Use `-std=c99` when building with CMake #331

Job	Run time
Installation tests	16s
Run static checkers	30s
macOS	1m 6s
Build with make	4m 37s
Windows	4m 10s
Windows (MinGW64)	55s
Build with CMake [Release, gcc-14, cmake-3.29, sanitizer=""]	5m 45s
Build with CMake [Release, gcc-14, cmake-3.29, sanitizer="thread"]	6m 10s
Build with CMake [Release, gcc-14, cmake-3.29, sanitizer="undefined"]	6m 1s
Build with CMake [Release, gcc-14, cmake-3.29, sanitizer="leak"]	5m 43s
Build with CMake [Release, gcc-14, cmake-3.29, sanitizer="address"]	5m 54s
Build with CMake [Release, clang-18, cmake-3.29, sanitizer=""]	5m 9s
Build with CMake [Release, clang-18, cmake-3.29, sanitizer="thread"]	4m 59s
Build with CMake [Release, clang-18, cmake-3.29, sanitizer="undefined"]	4m 57s
Build with CMake [Release, clang-18, cmake-3.29, sanitizer="leak"]	5m 12s
Build with CMake [Release, clang-18, cmake-3.29, sanitizer="address"]	4m 56s
Build with CMake [Debug, clang-18, cmake-3.29, sanitizer=""]	4m 48s
Build with CMake [Release, gcc-7, cmake-3.13, sanitizer=""]	4m 21s
Build with CMake [Release, clang-12, cmake-3.13, sanitizer=""]	5m 36s
	1h 21m 5s

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