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

configure.ac

@@ -710,6 +710,9 @@ case $host in
      fi
 
      CORE_CPPFLAGS="$CORE_CPPFLAGS -D_MT -DWIN32 -D_WINDOWS -D_WIN32_WINNT=0x0601 -D_WIN32_IE=0x0501 -DWIN32_LEAN_AND_MEAN"
+     dnl Prevent the definition of min/max macros.
+     dnl We always want to use the standard library.
+     CORE_CPPFLAGS="$CORE_CPPFLAGS -DNOMINMAX"
 
      dnl libtool insists upon adding -nostdlib and a list of objects/libs to link against.
      dnl That breaks our ability to build dll's with static libgcc/libstdc++/libssp. Override

src/compat.h

@@ -11,9 +11,6 @@
 #endif
 
 #ifdef WIN32
-#ifndef NOMINMAX
-#define NOMINMAX
-#endif
 #ifdef FD_SETSIZE
 #undef FD_SETSIZE // prevent redefinition compiler warning
 #endif

src/fs.cpp

@@ -12,9 +12,6 @@
 #include <sys/utsname.h>
 #include <unistd.h>
 #else
-#ifndef NOMINMAX
-#define NOMINMAX
-#endif
 #include <codecvt>
 #include <limits>
 #include <windows.h>

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>

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

src/util/system.cpp

@@ -62,9 +62,6 @@
 #pragma warning(disable:4717)
 #endif
 
-#ifndef NOMINMAX
-#define NOMINMAX
-#endif
 #include <codecvt>
 
 #include <io.h> /* for _commit */