Add x86_64 w64 mingw32 target courtesy of dukrat.

configure.ac

@@ -76,6 +76,13 @@ case $target in
 esac
 
 case $target in
+	x86_64-w64-mingw32)
+	have_x86_64=true
+	have_win32=true
+	PTHREAD_FLAGS=""
+	DLOPEN_FLAGS=""
+	WS2_LIBS="-lws2_32"
+	;;
   *-*-mingw*)
     have_x86_64=false
     have_win32=true