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, 10 insertions, 0 deletions

diff --git a/scripts/bundle.lua b/scripts/bundle.lua
index 07e120d04..b2a5e1e08 100644
--- a/scripts/bundle.lua
+++ b/scripts/bundle.lua
@@ -17,7 +17,15 @@ end
 
 --------------------------------------------------------------------------------
 local function _build(arch, debug, config_args)
+    import("core.project.config")
+    config.load()
+
     variant = debug and "debug" or "release"
+
+    -- Preserve toolchain/sdk from current config so --clean doesn't lose them
+    local toolchain_arg = config.get("toolchain") and ("--toolchain=" .. config.get("toolchain")) or nil
+    local sdk_arg = config.get("sdk") and ("--sdk=" .. config.get("sdk")) or nil
+
     local ret = _exec(
         "xmake",
         "config",
@@ -26,6 +34,8 @@ local function _build(arch, debug, config_args)
         "--mode="..variant,
         "--arch="..arch,
         "--zensentry=yes",
+        toolchain_arg,
+        sdk_arg,
         config_args)
     if ret > 0 then
         raise("Failed to configure xmake")