Adding mac homebrew support to the cmake configuration

[djw8605]

No description provided.

[bbockelm]

Not entirely clear that's the best way to find libcrypto on OS X. Regardless, merging it to move forward.

Can you google around a little bit to see if there are other solutions that don't involve hardcoding?

[djw8605]

I googled around a ton to find how to link to openssl and get headers. I couldn't find anything helpful. Everything suggested installed homebrew.

The final libraries though do link to the system's libcrypto:

$ otool -L libSciTokens.0.dylib 
libSciTokens.0.dylib:
	/Users/derekweitzel/git/scitokens-cpp/build/libSciTokens.0.dylib (compatibility version 0.0.0, current version 0.0.2)
	/usr/lib/libcrypto.35.dylib (compatibility version 36.0.0, current version 36.0.0)
	/usr/lib/libcurl.4.dylib (compatibility version 7.0.0, current version 9.0.0)
	/usr/lib/libsqlite3.dylib (compatibility version 9.0.0, current version 274.20.0)
	/usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 400.9.4)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1252.200.5)

So the binaries are distributable.