Fix GCC -Wpedantic and -Wunused-but-set-variable warnings

[Joel Kiptoo (Kiptoo-Deus)]

Fixes #342.

GCC reports ISO C++ prohibits anonymous structs [-Wpedantic] for the anonymous structs in the XMFLOAT*, XMINT*, XMUINT* and PackedVector types. Clang's equivalent warnings are already suppressed by the existing #pragma clang diagnostic blocks in DirectXMath.h and DirectXPackedVector.h; this adds matching #pragma GCC diagnostic blocks (guarded by defined(__GNUC__) && !defined(__clang__), so clang is unaffected) inside them.

The anonymous struct in XMMATRIX (_XM_NO_INTRINSICS_ only) is outside those blocks, and clang warns about it too (-Wgnu-anonymous-struct, -Wnested-anon-types), so it gets its own push/pop for both compilers.

In XMMatrixDecompose, only cc of the second XM3RANKDECOMPOSE is used, so aa and bb are marked (void) to fix -Wunused-but-set-variable.

Testing

Built a small program including DirectXMath.h, DirectXPackedVector.h, DirectXCollision.h and DirectXColors.h (system headers first, sal.h from dotnet/runtime, per the README) with -Wall -Wextra -Wpedantic -O2, for C++14, C++17 and C++20, with GCC 15.1 and Apple clang 21 on macOS. It calls XMMatrixDecompose on a known scale/rotation/translation and checks the result.

Warnings from this change's categories (-Wpedantic, -Wunused-but-set-variable), same for all three standards:

	GCC before	GCC after	clang before	clang after
arm64 (NEON)	36	0	0	0
x86_64 (SSE)	36	0	0	0
_XM_NO_INTRINSICS_	37	0	2	0

All builds pass and the program returns the expected result. Anonymous structs in user code after the includes still warn with both compilers, so the suppression does not leak past the headers.

I don't have MSVC available to test with; the change only adds preprocessor blocks that MSVC skips, and the (void) casts.

Not included

GCC also reports -Wstrict-aliasing ("dereferencing type-punned pointer will break strict-aliasing rules"), which is not in this issue: 2 sites in DirectXPackedVector.inl (e.g. reinterpret_cast<float*>(&Result)[0] in XMConvertHalfToFloat) on the x86_64 and no-intrinsics paths, and 24 on the arm64 NEON path (DirectXMathVector.inl, DirectXMathConvert.inl, DirectXMath.h). These are real type-punning rather than style warnings, so I left them out of this PR; I'm happy to open a separate issue or PR if you'd like them addressed, and in which style (memcpy or otherwise).

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[Chuck Walbourn (walbourn)]

Why not use #elif defined(__GNUC__) ?

[Joel Kiptoo (Kiptoo-Deus)]

Makes sense, since clang defines __GNUC__ too, the #elif does the same job with less noise. Changed it everywhere.

[Chuck Walbourn (walbourn)]

This looks good.

Is there not a more specific warning we can use here for GCC or do we have to turn off all 'pedantic' warnings?

[Joel Kiptoo (Kiptoo-Deus)]

Not that I could find, unfortunately. On GCC 15 it's just reported as ISO C++ prohibits anonymous structs [-Wpedantic], with no separate flag for it.

The only narrower thing I know of is sticking __extension__ in front of each anonymous struct, but that's 35 of them between DirectXMath.h and DirectXPackedVector.h, and clang would still need its pragmas anyway. So I kept the GCC push/pop limited to the same blocks that already have the clang ones.