Define NOMINMAX to avoid min() and max() macros.

Evil windows headers define min() and max() as macros, causing problems
when trying to use those identifiers for unrelated functions, including
some in the standard C++ library. Previously I had added #undefs
in graph.h, which is the only place where it was causing trouble.
Now it's also failing in qdatetime.h and other places; I don't know why
it wasn't failing there before.

So I'll take a more global approach: define NOMINMAX via the build system.
That makes the Windows headers not define these evil macros to begin with.