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:
73cadc06c6/mingw-w64-headers/include/ntdef.h (L289-L300)

src/support/lockedpool.cpp

@@ -10,9 +10,6 @@
 #endif
 
 #ifdef WIN32
-#ifndef NOMINMAX
-#define NOMINMAX
-#endif
 #include <windows.h>
 #else
 #include <sys/mman.h> // for mmap