win32: use GCC ompiler flags on MinGW

Author: illwieckz

cmake/NaClFlags.cmake

@@ -39,7 +39,7 @@ macro(set_linker_flag FLAG)
 endmacro()
 
 # From MakeUnixLikeEnv() from (root)/SConstruct
-if (LINUX OR APPLE)
+if (LINUX OR APPLE OR MINGW)
 	set_c_flag("-std=gnu99")
 	set_c_flag("-Wdeclaration-after-statement")
 	set_c_flag("-Wstrict-prototypes")