Set C standard at 99 for all test targets

Related to #1593.

tests/CMakeLists.txt

@@ -20,6 +20,18 @@ set(GETOPT "${GLFW_SOURCE_DIR}/deps/getopt.h"
 set(TINYCTHREAD "${GLFW_SOURCE_DIR}/deps/tinycthread.h"
                 "${GLFW_SOURCE_DIR}/deps/tinycthread.c")
 
+if (${CMAKE_VERSION} VERSION_EQUAL "3.1.0" OR
+    ${CMAKE_VERSION} VERSION_GREATER "3.1.0")
+    set(CMAKE_C_STANDARD 99)
+else()
+    # Remove this fallback when removing support for CMake version less than 3.1
+    add_compile_options(
+                        "$<$<C_COMPILER_ID:AppleClang>:-std=c99>"
+                        "$<$<C_COMPILER_ID:Clang>:-std=c99>"
+                        "$<$<C_COMPILER_ID:GNU>:-std=c99>")
+
+endif()
+
 add_executable(clipboard clipboard.c ${GETOPT} ${GLAD_GL})
 add_executable(events events.c ${GETOPT} ${GLAD_GL})
 add_executable(msaa msaa.c ${GETOPT} ${GLAD_GL})