build-windows: Update for msys

Building under msys2(https://msys2.github.io/) revealed a small bug while
defining the path separator.

Signed-off-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Acked-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

m4/absolute-header.m4

@@ -68,7 +68,7 @@ AC_DEFUN([gl_ABSOLUTE_HEADER_ONE],
   esac
 changequote(,)
   case "$host_os" in
-    mingw*)
+    (mingw*|msys*)
       dnl For the sake of native Windows compilers (excluding gcc),
       dnl treat backslash as a directory separator, like /.
       dnl Actually, these compilers use a double-backslash as