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.

test/macos_vs_bsd.c

@@ -0,0 +1,20 @@
+/*
+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)
+*/
+
+/*
+ * OSX can masquerade as BSD when sys/param.h is previously included.
+ * So we test that we only detect OSX in this combination.
+ */
+#include <boost/predef/os/macos.h>
+
+#if BOOST_OS_MACOS
+#   include <sys/param.h>
+#   include <boost/predef/os/bsd.h>
+#   if BOOST_OS_BSD
+#       error "Both BOOST_OS_MACOS and BOOST_OS_BSD defined."
+#   endif
+#endif