Yosemite build alternative fix.

This commit should fix the build for Yosemite OS, which was broken
due to the introduction of conflicting macros.

PR #77 fix the build.
This should also fix the build, but in a less intrusive way.

It prevents the definition of the htonll and ntohll function
inside the zmqpp namespace if a equivalent macro is present.
message.cpp has been update so that it will either call the macro
or the zmqpp function.

However, it is yet to be tested on Yosemite.
Merge pull request #78 from xaqq/yosemite-fix

Yosemite build alternative fix.