Clean up and document our hacks for building BSD source.

Change-Id: Ic591e22fa5b363bb68376b9f25814c0e5bd83fbf

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

@@ -0,0 +1,54 @@
+/*
+ * Copyright (C) 2013 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_FREEBSD_COMPAT_H_included
+#define _BIONIC_FREEBSD_COMPAT_H_included
+
+#define __USE_BSD
+#define REPLACE_GETOPT
+
+/*
+ * FreeBSD's libc has three symbols for every symbol:
+ *
+ *     __f will be the actual implementation.
+ *     _f will be a weak reference to __f (used for calls to f from within the library).
+ *     f will be a weak reference to __f (used for calls to f from outside the library).
+ *
+ * We collapse this into just the one symbol, f.
+ */
+
+/* Prevent weak reference generation. */
+#define __weak_reference(sym,alias)
+
+/* Ensure that the implementation itself gets the underscore-free name. */
+#define __sleep sleep
+#define __usleep usleep
+
+/* Redirect internal C library calls to the public function. */
+#define _close close
+#define _fcntl fcntl
+#define _fstat fstat
+#define _nanosleep nanosleep
+#define _open open
+
+/* This one is only needed as long as we have a mix of OpenBSD and FreeBSD stdio. */
+#define _sseek __sseek
+
+/* This is in BSD's <stdlib.h>. */
+#include <stdint.h>
+extern uint32_t arc4random_uniform(uint32_t upper_bound);
+
+#endif