OS X compatibility cleanups.

Reattach the licenses for the compat stuff to the source.
Only build/link the compat stuff when it's needed.
Rename compat/util.h so it's not confused with util.h.
Move all the platform specific defines inside compat/compat.h.

compat/compat.h

@@ -0,0 +1,32 @@
+#ifndef COMPAT_UTIL_H
+#define COMPAT_UTIL_H
+
+/*
+ * Platform-specific bits
+ */
+
+#if defined(__APPLE__)
+
+/* implementations of clock_gettime, clock_nanosleep */
+
+#include "clock_gettime/clock_gettime.h"
+#include "clock_nanosleep/clock_nanosleep.h"
+
+/*
+ * Mach endian conversion
+ */
+# include <libkern/OSByteOrder.h>
+# define bswap_16 OSSwapInt16
+# define bswap_32 OSSwapInt32
+# define bswap_64 OSSwapInt64
+# include <machine/endian.h>
+# define le16toh(x) OSSwapLittleToHostInt16(x)
+# define le32toh(x) OSSwapLittleToHostInt32(x)
+
+#else // other platforms
+
+# include <endian.h>
+
+#endif
+
+#endif //COMPAT_UTIL_H