Clean up and document our hacks for building BSD source.

Change-Id: Ic591e22fa5b363bb68376b9f25814c0e5bd83fbf

libc/upstream-netbsd/android/include/netbsd-compat.h

@@ -0,0 +1,34 @@
+/*
+ * Copyright (C) 2012 The Android Open Source Project
+ *
+ * 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.
+ */
+
+#ifndef _BIONIC_NETBSD_COMPAT_H_included
+#define _BIONIC_NETBSD_COMPAT_H_included
+
+// NetBSD uses _DIAGASSERT to null-check arguments and the like.
+#include <assert.h>
+#define _DIAGASSERT(e) ((e) ? (void) 0 : __assert2(__FILE__, __LINE__, __func__, #e))
+
+// TODO: update our <sys/cdefs.h> to support this properly.
+#define __type_fit(t, a) (0 == 0)
+
+#define _GNU_SOURCE
+#define __USE_BSD
+
+// TODO: we don't yet have thread-safe environment variables.
+#define __readlockenv() 0
+#define __unlockenv() 0
+
+#endif