CMake: Use C99 to build rcheevos

dep/rcheevos/CMakeLists.txt

@@ -46,6 +46,12 @@ target_include_directories(rcheevos PRIVATE "${CMAKE_CURRENT_SOURCE_DIR}/include
 target_include_directories(rcheevos INTERFACE "${CMAKE_CURRENT_SOURCE_DIR}/include")
 target_compile_definitions(rcheevos PRIVATE "RCHEEVOS_URL_SSL=1" "RC_NO_THREADS=1")
 
+# Use C99.
+set_target_properties(rcheevos PROPERTIES
+  C_STANDARD 99
+  C_STANDARD_REQUIRED ON
+)
+
 # RAIntegration is not supported outside of Win32 and only on x64.
 if(WIN32 AND CPU_ARCH_X64)
   target_sources(rcheevos PRIVATE