BUILD: add explicit support for Mac OS/X

The "osx" target may now be passed in the TARGET variable. It supports
the same features as FreeBSD and allows its users to use the GNU makefile
instead of the platform-specific makefile which lacks some features.
diff --git a/README b/README
index 0da3720..01cb99d 100644
--- a/README
+++ b/README
@@ -28,6 +28,7 @@
   - linux2628   for Linux 2.6.28 and above (enables splice and tproxy)
   - solaris     for Solaris 8 or 10 (others untested)
   - freebsd     for FreeBSD 5 to 8.0 (others untested)
+  - osx         for Mac OS/X
   - openbsd     for OpenBSD 3.1 to 5.2 (others untested)
   - aix52       for AIX 5.2
   - cygwin      for Cygwin