build: win: detect also mingw64 in msys2 setup

MSYS2 installs 3 environments, with uname (e.g. on win8.1 64) as follows:
- MINGW32_NT-6.3  gcc -> stand-alone native i686 binaries
- MINGW64_NT-6.3  gcc -> stand-alone native x86_64 binaries
- MSYS_NT-6.3     gcc -> posix-ish binaries which can only run in this env

Therefore 'MINGW' is more generic and detects both 32/64 native
environments, where previously 'MINGW32' detected only the 32 one.

configure

@@ -49,7 +49,7 @@ cpu=
 # OS specific
 targetos=`uname`
 case $targetos in
-  MINGW32*)  mingw32=yes;;
+  MINGW*)    mingw32=yes;;
   MSYS*)     mingw32=yes;;
   DragonFly) noldl=yes;;
   OpenBSD)   noldl=yes;;