bug#48303: 28.0.50; Build of master fails on MSYS2 MINGW64 environment

[Yasuhiro Kimura]

Environment:

* 64bit Windows 20H2
* MSYS2 MINGW64 environment
  ** mingw-w64-x86_64-gcc 10.3.0-2
  ** mingw-w64-x86_64-giflib 5.2.1-3
  ** mingw-w64-x86_64-gnutls 3.7.0-2
  ** mingw-w64-x86_64-libjpeg-turbo 2.0.6-1
  ** mingw-w64-x86_64-libpng 1.6.37-4
  ** mingw-w64-x86_64-librsvg 2.50.3-1
  ** mingw-w64-x86_64-libtiff 4.2.0-3
  ** mingw-w64-x86_64-libxml2 2.9.10-8
  ** mingw-w64-x86_64-xpm-nox 4.2.0-5
  ** mingw-w64-x86_64-zlib 1.2.11-9
* bb8b8d717f of master

Build Steps:

1. ./autogen.sh
2. ./configure
3. make

Result:

  CC       w32reg.o
  CC       w32font.o
  CC       w32term.o
  CC       w32xfns.o
  CC       w32select.o
  CC       w32uniscribe.o
  CC       w32cygwinx.o
  CC       w32.o
w32.c: In function '_sys_read_ahead':
w32.c:8853:10: warning: 'rc' may be used uninitialized in this function 
[-Wmaybe-uninitialized]
 8853 |   if (rc == sizeof (char))
      |       ~~~^~~~~~~~~~~~~~~~
  CC       w32console.o
  CC       w32heap.o
  CC       w32inevt.o
  CC       w32proc.o
  CC       w32image.o
In file included from w32image.c:32:
w32common.h:45:3: error: expected identifier before numeric constant
   45 |   OS_NT
      |   ^~~~~
make[1]: *** [Makefile:385: w32image.o] Error 1
make[1]: Leaving directory '/c/Users/yasu/Work/Emacs/master/emacs/src'
make: *** [Makefile:425: src] Error 2
(MINGW64)yasu@rolling[1071]%