fix extern prefix detection on mingw

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@12485 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

configure

@@ -1494,7 +1494,7 @@ fi
 check_cc <<EOF || die "Symbol mangling check failed."
 int ff_extern;
 EOF
-sym=$(nm -P $TMPO)
+sym=$(nm -P -g $TMPO)
 extern_prefix=${sym%%ff_extern*}
 
 check_asm inline_asm '""'