Add a kerberos portability header

Summary: Kerberos has issues with include order on Windows, so give it it's own portability header to force the correct include order and undefine the mess it `#define`'s.

Reviewed By: yfeldblum

Differential Revision: D8050238

fbshipit-source-id: 6c718f8f0db9f039734bead5f90cd289ea1dfd78

folly/portability/Krb5.h

+/*
+ * Copyright 2016-present Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+// Bring in MAXPATHLEN so kerberos doesn't make up its
+// own defintion of it.
+#include <folly/portability/Stdlib.h>
+
+#include <folly/portability/Windows.h>
+
+#include <gssapi/gssapi_generic.h>
+#include <gssapi/gssapi_krb5.h>
+#include <krb5.h> // @manual
+
+// Kerberos defines a bunch of things that we implement as actual
+// functions, so undefine whatever mess kerberos has done.
+#undef strcasecmp
+#undef strncasecmp
+#undef strdup
+#undef strtok_r