fixed #562 - avoid potential redefinition of Windows-specific macros (#563)

Author: firewave

simplecpp.cpp

@@ -7,8 +7,12 @@
 #  ifndef _WIN32_WINNT
 #    define _WIN32_WINNT 0x0602
 #  endif
-#  define NOMINMAX
-#  define WIN32_LEAN_AND_MEAN
+#  ifndef NOMINMAX
+#    define NOMINMAX
+#  endif
+#  ifndef WIN32_LEAN_AND_MEAN
+#    define WIN32_LEAN_AND_MEAN
+#  endif
 #  include <windows.h>
 #  undef ERROR
 #endif