Enable C99 explicitly with CMake where available

This enables compilation as C99 where supported by the compiler.
A workaround with per-compiler hardcoded flags is used for CMake 3.0,
which does not support the C_STANDARD target property.

Fixes #1560.
Closes #1576.

src/CMakeLists.txt

@@ -100,6 +100,16 @@ set_target_properties(glfw PROPERTIES
                       POSITION_INDEPENDENT_CODE ON
                       FOLDER "GLFW3")
 
+if (${CMAKE_VERSION} VERSION_GREATER_EQUAL "3.1.0")
+    set_target_properties(glfw PROPERTIES C_STANDARD 99)
+else()
+    # Remove this fallback when removing support for CMake version less than 3.1
+    target_compile_options(glfw PRIVATE
+                           "$<$<C_COMPILER_ID:AppleClang>:-std=c99>"
+                           "$<$<C_COMPILER_ID:Clang>:-std=c99>>"
+                           "$<$<C_COMPILER_ID:GNU>:-std=c99>>")
+endif()
+
 target_compile_definitions(glfw PRIVATE _GLFW_USE_CONFIG_H)
 target_include_directories(glfw PUBLIC
                            "$<BUILD_INTERFACE:${GLFW_SOURCE_DIR}/include>"