Linking against libc++ instead of the GNU libstdc++ on OSX.

[zoq]

Since we are using C++11 features we link against libstdc++ when using clang. libstdc++ is tried-and-tested, but it is a dead end on OSX that results in some problems like:

error: no template named 'forward' in namespace 'std';

Due to Apple not upgrading past GCC 4.2 for license reasons, Apple switched the standard library, so clang++ started to link against libc++ rather than libstdc++. The good news is that libc++ is specifically designed so that we can use it in the same process as libstdc++.

[zoq]

Patched in 8aa3abb

@saketkc This should also fix the homebrew science build error. If you like we can create a patch against mlpack 1.0.12.

[saketkc]

Thanks, @zoq. I will update it.

[rcurtin]

Issue appears fixed, so I'm closing it. If I shouldn't do that, then it can be reopened. :)