project: build in (at least) c99 mode

CMakeLists.txt

@@ -381,12 +381,14 @@ if(SDL_SHARED)
   add_library(SDL3-shared SHARED)
   add_library(SDL3::SDL3-shared ALIAS SDL3-shared)
   SDL_AddCommonCompilerFlags(SDL3-shared)
+  target_compile_features(SDL3-shared PRIVATE c_std_99)
 endif()
 
 if(SDL_STATIC)
   add_library(SDL3-static STATIC)
   add_library(SDL3::SDL3-static ALIAS SDL3-static)
   SDL_AddCommonCompilerFlags(SDL3-static)
+  target_compile_features(SDL3-static PRIVATE c_std_99)
 endif()
 
 if(SDL_TEST)