Add x86_64 w64 mingw32 target courtesy of dukrat.
diff --git a/configure.ac b/configure.ac
index e53fa5c..3bebfaf 100644
--- a/configure.ac
+++ b/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