cmake: fix sdlcpu.cmake for C89 compilers #75

Job	Run time
Create test plan	4s
level1 / Haiku	1m 56s
level2 / Ubuntu 22.04	4m 47s
level2 / Android (Android.mk)	1m 17s
level2 / PowerPC	1m 47s
level2 / Windows (msys2, ucrt64)	8m 18s
level2 / Windows (MSVC, clang-cl x64)	5m 8s
level2 / MacOS (Framework) (arm64)	2m 40s
level2 / Nintendo 3DS	1m 52s
level2 / Sony PlayStation Vita (GLES w/ PVR_PSP2)	3m 14s
level2 / Ubuntu 20.04 (Intel oneAPI)	6m 47s
level2 / RISC OS	1m 13s
level2 / Windows (msys2, clang32)	6m 34s
level2 / Windows (MSVC, x64)	4m 46s
level2 / Windows (MSVC, x86)	4m 59s
level2 / Android (CMake, lean)	2m 46s
level2 / Sony PlayStation 2	4m 47s
level2 / Windows (MSVC, clang-cl x86)	5m 4s
level2 / Steam Linux Runtime (Sniper)	2m 55s
level2 / MacOS (Framework) (x64)	8m 2s
level2 / Windows (msys2, clang64)	5m 51s
level2 / Sony PlayStation Portable	3m 48s
level2 / Windows (msys2, mingw64)	8m 10s
level2 / LoongArch64	2m 55s
level2 / FreeBSD	2m 43s
level2 / Android (CMake)	2m 59s
level2 / Emscripten	9m 10s
level2 / PowerPC64	1m 48s
level2 / Ubuntu 20.04 (Intel Compiler)	4m 58s
level2 / GDK (MSVC, x64)	3m 19s
level2 / MacOS (GNU prefix)	4m 50s
level2 / Windows (MSVC, ARM64)	3m 13s
level2 / Windows (MSVC, ARM)	3m 19s
level2 / Android (Gradle)	2m 17s
level2 / tvOS (CMake & xcode)	1m 57s
level2 / Sony PlayStation Vita (GLES w/ pib)	3m 8s
level2 / NetBSD	2m 28s
level2 / Windows (msys2, mingw32)	8m 16s
level2 / Ubuntu 20.04	4m 7s
level2 / iOS (CMake & xcode)	2m 34s
	2h 40m 46s

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