title: Unknown OS 'msys_nt-6.1'. using Mingw on win7 64bit
body: <p>I try to compile FFmpeg from source using MSYS2  and MIngW I'm using win7 64 bit .
  using .</p>
  
  <pre><code>$ ./configure --enable-shared
  </code></pre>
  
  <p>I'm getting this error:</p>
  
  <pre><code>Unknown OS 'msys_nt-6.1'.
  
  If you think configure made a mistake, make sure you are using the latest
  version from Git.  If the latest version fails, report the problem to the
  ffmpeg-user@ffmpeg.org mailing list or IRC #ffmpeg on irc.freenode.net.
  Include the log file "config.log" produced by configure as this will help
  solve the problem.
  </code></pre>
  
  <p>looking in to config.log 
  i see ( the end of the log ):</p>
  
  <pre><code>gcc -std=c99 -Wall -O3 -c -o /tmp/ffconf.fnJeLuIu.o /tmp/ffconf.e8DJYGJM.c
  check_code cc  int test[2*(sizeof(void *) &gt; 4) - 1]
  check_cc
  BEGIN /tmp/ffconf.e8DJYGJM.c
      1   int main(void) { int test[2*(sizeof(void *) &gt; 4) - 1]; return 0; }
  END /tmp/ffconf.e8DJYGJM.c
  gcc -D_ISOC99_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -std=c99 -c -o /tmp/ffconf.fnJeLuIu.o /tmp/ffconf.e8DJYGJM.c
  G:/msys64/tmp/ffconf.e8DJYGJM.c: In function 'main':
  G:/msys64/tmp/ffconf.e8DJYGJM.c:1:22: error: size of array 'test' is negative
   int main(void) { int test[2*(sizeof(void *) &gt; 4) - 1]; return 0; }
                        ^
  check_cpp_condition stddef.h defined(__x86_64__)
  check_cpp
  BEGIN /tmp/ffconf.e8DJYGJM.c
      1   #include &lt;stddef.h&gt;
      2   #if !(defined(__x86_64__))
      3   #error "unsatisfied condition: defined(__x86_64__)"
      4   #endif
  END /tmp/ffconf.e8DJYGJM.c
  gcc -D_ISOC99_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -std=c99 -E -o /tmp/ffconf.fnJeLuIu.o /tmp/ffconf.e8DJYGJM.c
  G:/msys64/tmp/ffconf.e8DJYGJM.c:3:2: error: #error "unsatisfied condition: defined(__x86_64__)"
   #error "unsatisfied condition: defined(__x86_64__)"
    ^
  Unknown OS 'msys_nt-6.1'. 
  </code></pre>
  
  <p>what did i miss ? what did i do wrong ?
  Thanks</p>