the feature config files are generated without inclusion guards.

this causes issues with the SIMD detection code for universal binaries on macos:

• qconfig.h defines e.g. QT_COMPILER_SUPPORTS_SSE2
• qcompilerdetection.h potentially undefines it again
• later inclusions of qconfig.h re-defines it again.

on macos one should typically rely on the compiler-provided preprocessor defines like _SSE_, as they are populated based on the minimum deployment target.