build: globally define NOMINMAX

Define (and document) `NOMINMAX` once, rather than across multiple
source files.

Defining this prevents the definition of min/max macros when using
mingw-w64, which may conflict with unprefixed std::min/max usage. While
that might not be the case for us, we'd always prefer to use the standard
library in any case.

For example:
https://github.com/mingw-w64/mingw-w64/blob/73cadc06c62c6af5faf76f64ef08e684b48de48c/mingw-w64-headers/include/ntdef.h#L289-L300

src/qt/guiutil.cpp

@@ -24,9 +24,6 @@
 #include <util/time.h>
 
 #ifdef WIN32
-#ifndef NOMINMAX
-#define NOMINMAX
-#endif
 #include <shellapi.h>
 #include <shlobj.h>
 #include <shlwapi.h>