dtoolbase: fix NOMINMAX macro redefinition warning

dtool/src/dtoolbase/dtoolbase.h

@@ -63,8 +63,10 @@
 /* Windows likes to define min() and max() macros, which will conflict with
    std::min() and std::max() respectively, unless we do this: */
 #ifdef WIN32
+#ifndef NOMINMAX
 #define NOMINMAX
 #endif
+#endif
 
 #ifndef __has_builtin
 #define __has_builtin(x) 0