Address endian compile errors for OSX in the cases where sys/param.h was

include ahead of the predef headers. The solution was to: use the OSX
machine/endian.h header, and only allow a single OS to be detected at
once. Test cases added for both conditions on OSX.

include/boost/predef/detail/os_detected.h

@@ -0,0 +1,10 @@
+/*
+Copyright Redshift Software, Inc. 2013
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE_1_0.txt or copy at
+http://www.boost.org/LICENSE_1_0.txt)
+*/
+
+#ifndef BOOST_PREDEF_DETAIL_OS_DETECTED
+#define BOOST_PREDEF_DETAIL_OS_DETECTED 1
+#endif