native xmake toolchain definition for UE-clang (#805)

This change is meant to provide a smoother experience when working on Linux. After this change, the toolchain setup process is now simply

```bash
$ scripts/ue_build_linux/get_ue_toolchain.sh 
```

and then at config time the toolchain is automatically detected if you downloaded it to the default location or have the `UE_TOOLCHAIN_DIR` environment variable set

```bash
xmake config --mode=debug
```

Compared to the old script-based approach this configures the toolchain more precisely, avoiding leakage into unrelated build processes such as when a package manager decides to build something like Ninja locally etc.

1 files changed, 32 insertions, 10 deletions

diff --git a/xmake.lua b/xmake.lua
index 0d8f53a16..7b9fee594 100644
--- a/xmake.lua
+++ b/xmake.lua
@@ -101,16 +101,17 @@ if is_plat("windows") then
     add_requires("7z")
 end
 
--- If we're using the UE cross-compile toolchain, we need to ensure we link statically
--- against the toolchain libc++ and libc++abi, as the system ones can differ in ABI
--- leading to nasty crashes
-
-if is_plat("linux") and os.getenv("UE_TOOLCHAIN_DIR") then
-    add_ldflags("-static-libstdc++")
-    add_ldflags("$(projectdir)/thirdparty/ue-libcxx/lib64/libc++.a")
-    add_ldflags("$(projectdir)/thirdparty/ue-libcxx/lib64/libc++abi.a")
-    set_toolset("objcopy", "$(env UE_TOOLCHAIN_DIR)/bin/llvm-objcopy")
+-- When using the UE clang toolchain, statically link the toolchain's libc++ and
+-- libc++abi to avoid ABI mismatches with system libraries at runtime.
+-- These are project-level flags (not in the toolchain definition) so they don't
+-- propagate into cmake package builds.
+if is_plat("linux") and get_config("toolchain") == "ue-clang" then
+    add_ldflags("-static-libstdc++", {force = true})
+    add_ldflags("$(projectdir)/thirdparty/ue-libcxx/lib64/libc++.a", {force = true})
+    add_ldflags("$(projectdir)/thirdparty/ue-libcxx/lib64/libc++abi.a", {force = true})
+    add_ldflags("-lpthread", {force = true})
 end
+
 if has_config("zensentry") and not use_asan then
     if is_plat("linux") then
         add_requires("sentry-native 0.12.1", {configs = {backend = "crashpad"}})
@@ -276,6 +277,25 @@ set_languages("cxx20")
 -- always generate debug information
 set_symbols("debug")
 
+includes("toolchains")
+
+-- Auto-select the UE clang toolchain on Linux when the SDK is available
+if is_plat("linux") and not get_config("toolchain") then
+    local ue_sdk = os.getenv("UE_TOOLCHAIN_DIR")
+    if not ue_sdk or ue_sdk == "" then
+        local home = os.getenv("HOME")
+        if home then
+            local default_path = path.join(home, ".ue-toolchain")
+            if os.isdir(default_path) then
+                ue_sdk = default_path
+            end
+        end
+    end
+    if ue_sdk and ue_sdk ~= "" and os.isdir(ue_sdk) then
+        set_toolchains("ue-clang")
+    end
+end
+
 includes("thirdparty")
 includes("src/transports")
 includes("src/zenbase")
@@ -459,7 +479,9 @@ task("test")
 
         -- Only reconfigure if current config doesn't already match
         if config.get("mode") ~= "debug" or config.get("plat") ~= plat or config.get("arch") ~= arch then
-            os.exec("xmake config -c -m debug -p %s -a %s", plat, arch)
+            local toolchain_flag = config.get("toolchain") and ("--toolchain=" .. config.get("toolchain")) or ""
+            local sdk_flag = config.get("sdk") and ("--sdk=" .. config.get("sdk")) or ""
+            os.exec("xmake config -c -m debug -p %s -a %s %s %s", plat, arch, toolchain_flag, sdk_flag)
         end
 
         -- Build targets we're going to run