Enable cross compilation of Windows targets on Linux (#839)

This PR makes it *possible* to do a Windows build on Linux via `clang-cl`. 

It doesn't actually change any build process. No policy change, just mechanics and some code fixes to clear clang compilation.

The code fixes are mainly related to #include file name casing, to match the on-disk casing of the SDK files (via xwin).

1 files changed, 51 insertions, 47 deletions

diff --git a/xmake.lua b/xmake.lua
index 4110f25e2..846bc8f1d 100644
--- a/xmake.lua
+++ b/xmake.lua
@@ -6,6 +6,12 @@ set_configvar("ZEN_DATA_FORCE_SCRUB_VERSION", 0)
 set_allowedplats("windows", "linux", "macosx")
 set_allowedarchs("windows|x64", "linux|x86_64", "macosx|x86_64", "macosx|arm64")
 
+-- Returns true when building for Windows with native MSVC (not clang-cl cross-compilation)
+function is_native_msvc()
+    local tc = get_config("toolchain") or ""
+    return is_plat("windows") and tc ~= "clang-cl"
+end
+
 --------------------------------------------------------------------------
 -- We support debug and release modes. On Windows we use static CRT to
 -- minimize dependencies.
@@ -146,7 +152,9 @@ enable_unity = false
 if is_mode("release") then
     -- LTO does not appear to work with the current Linux UE toolchain
     -- Also, disabled LTO on Mac to reduce time spent building openssl tests
-    if not is_plat("linux", "macosx") then
+    -- Disabled for cross-compilation (clang-cl on Linux) due to cmake package compat issues
+    local is_cross_win = is_plat("windows") and is_host("linux")
+    if not is_plat("linux", "macosx") and not is_cross_win then
         set_policy("build.optimization.lto", true)
     end
     set_optimize("fastest")
@@ -169,6 +177,12 @@ else
     set_encodings("source:utf-8", "target:utf-8")
 end
 
+-- When cross-compiling with clang-cl, the xwin SDK may ship a newer MSVC STL
+-- than the host clang version supports. Bypass the version gate.
+if is_plat("windows") and not is_native_msvc() then
+    add_defines("_ALLOW_COMPILER_AND_STL_VERSION_MISMATCH")
+end
+
 if is_os("windows") then
     add_defines(
         "_CRT_SECURE_NO_WARNINGS",
@@ -181,59 +195,49 @@ if is_os("windows") then
         "_WIN32_WINNT=0x0A00",
         "_WINSOCK_DEPRECATED_NO_WARNINGS" -- let us use the ANSI functions
     )
-    -- Make builds more deterministic and portable
-    add_cxxflags("/d1trimfile:$(curdir)\\")     -- eliminates the base path from __FILE__ paths
-    add_cxxflags("/experimental:deterministic") -- (more) deterministic compiler output
-    add_ldflags("/PDBALTPATH:%_PDB%")           -- deterministic pdb reference in exe
-
-    add_cxxflags("/Zc:preprocessor")        -- Enable preprocessor conformance mode
-    add_cxxflags("/Zc:u8EscapeEncoding")    -- Enable UTF-8 encoding for u8 string literals
-    add_cxxflags("/Zc:inline")              -- Enforce inline semantics
+    -- Make builds more deterministic and portable (MSVC-only flags)
+    if is_native_msvc() then
+        add_cxxflags("/d1trimfile:$(curdir)\\")     -- eliminates the base path from __FILE__ paths
+        add_cxxflags("/experimental:deterministic") -- (more) deterministic compiler output
+        add_ldflags("/PDBALTPATH:%_PDB%")           -- deterministic pdb reference in exe
+        add_cxxflags("/Zc:u8EscapeEncoding")        -- Enable UTF-8 encoding for u8 string literals (clang does this by default)
+        add_cxxflags("/Zc:preprocessor")            -- Enable preprocessor conformance mode
+        add_cxxflags("/Zc:inline")                  -- Enforce inline semantics
+    end
     -- add_ldflags("/MAP")
 end
 
-if is_os("linux") or is_os("macosx") then
-    add_cxxflags("-Wno-implicit-fallthrough")
-    add_cxxflags("-Wno-missing-field-initializers")
-    add_cxxflags("-Wno-strict-aliasing")
-    add_cxxflags("-Wno-switch")
-    add_cxxflags("-Wno-unused-lambda-capture")
-    add_cxxflags("-Wno-unused-private-field")
-    add_cxxflags("-Wno-unused-value")
-    add_cxxflags("-Wno-unused-variable")
-    add_cxxflags("-Wno-vla-cxx-extension")
+-- Clang warning suppressions (native clang on Linux/Mac, or clang-cl cross-compile)
+if is_os("linux") or is_os("macosx") or not is_native_msvc() then
+    -- Silence warnings about unrecognized -Wno-* flags on older clang versions
+    add_cxxflags("-Wno-unknown-warning-option", {force = true})
+    add_cxxflags("-Wno-delete-non-abstract-non-virtual-dtor", {force = true})
+    add_cxxflags("-Wno-format", {force = true})
+    add_cxxflags("-Wno-implicit-fallthrough", {force = true})
+    add_cxxflags("-Wno-inconsistent-missing-override", {force = true})
+    add_cxxflags("-Wno-missing-field-initializers", {force = true})
+    add_cxxflags("-Wno-nonportable-include-path", {force = true})
+    add_cxxflags("-Wno-sign-compare", {force = true})
+    add_cxxflags("-Wno-strict-aliasing", {force = true})
+    add_cxxflags("-Wno-switch", {force = true})
+    add_cxxflags("-Wno-unused-lambda-capture", {force = true})
+    add_cxxflags("-Wno-unused-private-field", {force = true})
+    add_cxxflags("-Wno-unused-value", {force = true})
+    add_cxxflags("-Wno-unused-variable", {force = true})
+    add_cxxflags("-Wno-vla-cxx-extension", {force = true})
     -- GCC false positive: constinit static locals used by reference are reported as unused-but-set
-    add_cxxflags("-Wno-unused-but-set-variable", {tools="gcc"})
+    add_cxxflags("-Wno-unused-but-set-variable", {tools = "gcc"})
 end
 
+-- Additional suppressions specific to clang-cl cross-compilation
 if get_config("toolchain") == "clang-cl" then
-    add_cxxflags("-Wno-unknown-warning-option", {force=true})
-    add_cxxflags("-Wno-cast-function-type-mismatch", {force=true})
-    add_cxxflags("-Wno-delete-non-abstract-non-virtual-dtor", {force=true})
-    add_cxxflags("-Wno-format", {force=true})
-    add_cxxflags("-Wno-implicit-fallthrough", {force=true})
-    add_cxxflags("-Wno-inconsistent-missing-override", {force=true})
-    add_cxxflags("-Wno-missing-field-initializers", {force=true})
-    add_cxxflags("-Wno-parentheses-equality", {force=true})
-    add_cxxflags("-Wno-reorder-ctor", {force=true})
-    add_cxxflags("-Wno-sign-compare", {force=true})
-    add_cxxflags("-Wno-strict-aliasing", {force=true})
-    add_cxxflags("-Wno-switch", {force=true})
-    add_cxxflags("-Wno-unused-but-set-variable", {force=true})
-    add_cxxflags("-Wno-unused-lambda-capture", {force=true})
-    add_cxxflags("-Wno-unused-parameter", {force=true})
-    add_cxxflags("-Wno-unused-private-field", {force=true})
-    add_cxxflags("-Wno-unused-value", {force=true})
-    add_cxxflags("-Wno-unused-variable", {force=true})
-    add_cxxflags("-Wno-vla-cxx-extension", {force=true})
-    add_cflags("-Wno-unknown-warning-option", {force=true})
-    add_cflags("-Wno-unused-command-line-argument", {force=true})
-end
-
-if is_os("macosx") then
-    -- silence warnings about -Wno-vla-cxx-extension since to my knowledge we can't
-    -- detect the clang version used in Xcode and only recent versions contain this flag
-    add_cxxflags("-Wno-unknown-warning-option")
+    add_cxxflags("-Wno-cast-function-type-mismatch", {force = true})
+    add_cxxflags("-Wno-parentheses-equality", {force = true})
+    add_cxxflags("-Wno-reorder-ctor", {force = true})
+    add_cxxflags("-Wno-unused-but-set-variable", {force = true})
+    add_cxxflags("-Wno-unused-parameter", {force = true})
+    add_cflags("-Wno-unknown-warning-option", {force = true})
+    add_cflags("-Wno-unused-command-line-argument", {force = true})
 end
 
 if is_os("linux") then