Merge PR #1279: CMake: Explicitly set C standard to 99

CMakeLists.txt

@@ -11,6 +11,8 @@ project("SoftEther VPN"
   LANGUAGES C
 )
 
+set(CMAKE_C_STANDARD 99)
+
 set(TOP_DIRECTORY ${CMAKE_SOURCE_DIR})
 set(BUILD_DIRECTORY ${CMAKE_BINARY_DIR})