sysdefs.h: Update the comment about __USE_MINGW_ANSI_STDIO.

src/common/sysdefs.h

@@ -24,7 +24,15 @@
 #	include <config.h>
 #endif
 
-// Get standard-compliant stdio functions under MinGW and MinGW-w64.
+// This #define ensures that C99 and POSIX compliant stdio functions are
+// available with MinGW-w64 (both 32-bit and 64-bit). Modern MinGW-w64 adds
+// this automatically, for example, when the compiler is in C99 (or later)
+// mode when building against msvcrt.dll. It still doesn't hurt to be explicit
+// that we always want this and #define this unconditionally.
+//
+// With Universal CRT (UCRT) this is less important because UCRT contains
+// C99-compatible stdio functions. It's still nice to #define this as UCRT
+// doesn't support the POSIX thousand separator flag in printf (like "%'u").
 #ifdef __MINGW32__
 #	define __USE_MINGW_ANSI_STDIO 1
 #endif