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/Makefile b/Makefile
index da6efe9..b66a07f 100644
--- a/Makefile
+++ b/Makefile
@@ -269,6 +269,13 @@
   USE_TPROXY     = implicit
   USE_LIBCRYPT   = implicit
 else
+ifeq ($(TARGET),osx)
+  # This is for Mac OS/X
+  USE_POLL       = implicit
+  USE_KQUEUE     = implicit
+  USE_TPROXY     = implicit
+  USE_LIBCRYPT   = implicit
+else
 ifeq ($(TARGET),openbsd)
   # This is for OpenBSD >= 3.0
   USE_POLL       = implicit
@@ -291,6 +298,7 @@
 endif # cygwin
 endif # aix52
 endif # openbsd
+endif # osx
 endif # freebsd
 endif # solaris
 endif # linux2628