Configure: fix compilation on MSYS2 / MinGW64.

auto/configure

diff --git a/auto/configure b/auto/configure
index ceff15e..7e6e33a 100755
--- a/auto/configure
+++ b/auto/configure
@@ -36,7 +36,7 @@
     NGX_PLATFORM="$NGX_SYSTEM:$NGX_RELEASE:$NGX_MACHINE";
 
     case "$NGX_SYSTEM" in
-        MINGW32_*)
+        MINGW32_* | MINGW64_* | MSYS_*)
             NGX_PLATFORM=win32
         ;;
     esac