undefine mingw32 pseudo modifier IN

mingw32 defines pseudo modifiers IN, OUT, OPTIONAL. These
cause global conflicts if defined prior to using as a variable
name.

code reference: http://sourceforge.net/p/mingw-w64/code/6297/tree/stable/v3.x/mingw-w64-headers/include/minwindef.h#l56

Author: drewwells

src/units.cpp

@@ -1,6 +1,10 @@
 #include <stdexcept>
 #include "units.hpp"
 
+#ifdef IN
+#undef IN
+#endif
+
 namespace Sass {
 
   /* the conversion matrix can be readed the following way */