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Describe the bug

Filament cannot be built under MSYS2 (MinGW-w64) on Windows. While CMake configuration succeeds, the build fails at compile time due to:

1. The bluegl library requires the MASM assembler (ml/ml64.exe) which is not available under MSYS2.
2. Several source files use #if defined(WIN32) guards which are not triggered because MinGW Clang does not define the WIN32 macro (only _WIN32). This causes the wrong code paths to be taken throughout the codebase.

To Reproduce

1. Install MSYS2 with the UCRT64 environment:

   pacman -S mingw-w64-ucrt-x86_64-toolchain mingw-w64-ucrt-x86_64-clang cmake ninja

2. Clone Filament and configure:

   cmake -B build -G Ninja \
     -DCMAKE_C_COMPILER=clang \
     -DCMAKE_CXX_COMPILER=clang++

   Configuration succeeds.

3. Build:

   cmake --build build

   Build fails with multiple errors.

Expected behavior

Filament should either:

• Support building with MinGW-w64 (MSYS2) toolchains, or
• Explicitly reject MinGW at the compiler check stage with a clear error message.

Screenshots

N/A

Logs

--- bluegl: MASM assembler missing ---

[1/4] Building ASM_MASM object libs/bluegl/CMakeFiles/bluegl.dir/src/BlueGLCoreWindowsImpl.S.obj
FAILED: libs/bluegl/CMakeFiles/bluegl.dir/src/BlueGLCoreWindowsImpl.S.obj
ml ... -c -Fo ... D:/filament/libs/bluegl/src/BlueGLCoreWindowsImpl.S
CreateProcess failed: The system cannot find the file specified.

--- CallStack.cpp: execinfo.h not found (WIN32 not defined by MinGW Clang) ---

libs/utils/src/CallStack.cpp:31:10: fatal error: 'execinfo.h' file not found
   31 | #include <execinfo.h>

--- memalign.h: posix_memalign not found (WIN32 not defined by MinGW Clang) ---

libs/utils/include/utils/memalign.h:43:14: error: no member named 'posix_memalign' in the global namespace
   43 |     (void) ::posix_memalign(&p, align, size);

--- ashmem.cpp: MSVC-specific CRT functions not available ---

libs/utils/src/ashmem.cpp:155:35: error: cannot initialize a parameter of type 'wchar_t *'
    155 |     const char* tmpPath = _mktemp(template_path);
libs/utils/src/ashmem.cpp:156:29: error: use of undeclared identifier '_O_BINARY'
    156 |     int fd = _open(tmpPath, _O_BINARY);
libs/utils/src/ashmem.cpp:159:9: error: use of undeclared identifier '_chsize'
    159 |     if (_chsize(fd, size) == -1) {

Desktop (please complete the following information):

• OS: Windows 11 64-bit (MSYS2 UCRT64)
• GPU: NVIDIA GeForce RTX 4060 Max-Q (8GB VRAM)
• Backend: Vulkan

Additional context

Root cause: WIN32 vs _WIN32

Under MSVC, the compiler defines WIN32. Under MinGW Clang, only _WIN32, __WIN32, and __WIN32__ are defined — WIN32 (without underscore) is NOT defined:

$ echo 'int x;' | clang++ -std=c++20 -dM -E -x c++ - | grep -i win32
#define _WIN32 1
#define __WIN32 1
#define __WIN32__ 1

Filament uses #if defined(WIN32) in multiple places. Under MinGW these guards silently fail, causing the code to take Unix/POSIX code paths:

bluegl MASM requirement

libs/bluegl/CMakeLists.txt:13-16 enables MASM on 64-bit Windows:

if (WIN32 AND IS_64_BIT)
    enable_language(ASM_MASM)
    set_property(SOURCE src/BlueGLCoreWindowsImpl.S PROPERTY LANGUAGE ASM_MASM)
endif()

CMake finds a virtual ml entry but the binary does not exist in MSYS2, so it fails at build time with CreateProcess failed.

Why build on MSYS2?
I'm porting a cross-platform project to Windows, and the author made a MSYS2 build script. While it's possible to use MSVC to build the whole project, the build process just got failed on Filament, and other libraries have no issues.