windows: Fix compilation for MSYS2

Closes #140

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>
diff --git a/configure.ac b/configure.ac
index 4e35bf6..9f3e639 100644
--- a/configure.ac
+++ b/configure.ac
@@ -78,7 +78,7 @@
 	backend="netbsd"
 	threads="posix"
 	;;
-*-mingw*)
+*-mingw* | *msys*)
 	AC_MSG_RESULT([Windows])
 	backend="windows"
 	threads="windows"