Make the GCC deprecation-message check actually test the GCC version

[Chang-Jin-Lee]

Fixes #336.

GCC_VERSION is not predefined by GCC, so #if GCC_VERSION >= 40500 has always compared 0 against 40500 and taken the branch below it. Every deprecated symbol in the public header loses its message on GCC:

warning: ‘spvReflectGetShaderModule’ is deprecated [-Wdeprecated-declarations]

and with this change it reads

warning: ‘spvReflectGetShaderModule’ is deprecated: renamed to spvReflectCreateShaderModule [-Wdeprecated-declarations]

-Wundef names the cause on its own: "GCC_VERSION" is not defined, evaluates to 0, at spirv_reflect.h:49. Clang and MSVC were never affected, since they are handled by the branches above.

The issue suggests defining GCC_VERSION. I spelled the comparison out instead — this is a public header, and GCC_VERSION is a common enough spelling in other projects that defining it here could collide with a consumer's own. The >= 40500 gate is left alone rather than dropped, so the intent of the original code is unchanged.

Checked on GCC 13.3.0: the message appears in C and in C++ under -Wall -Wextra, the -Wundef warning is gone, and test-spirv-reflect is 3678 passing. The edit sits inside the file's clang-format off region, so the formatting job has nothing to reflow.

🤖 Generated with Claude Code

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[spencer-lunarg]

@Chang-Jin-Lee can you make sure the CLA is signed for this repo so I can merge this

(if you did, try https://cla-assistant.io/ to retrigger it)

[Chang-Jin-Lee]

Sorry for the slow reply. The CLA is mine to clear and it is still unsigned — license/cla is the only thing not green here, everything else passed. I am going through the signing flow now and will retrigger it as you suggested once it is done.