Fix some mac-specific build issues

Summary: In Makefiles, CPP is for the C preprocessor, not the c++ compiler, which is CXX. This was causing the library files to not compile at all, but simply go through the preprocessor step.

Test Plan: build/unit tests (mac)

Reviewers: strangemonad

Reviewed By: strangemonad

Subscribers: team

Differential Revision: http://phabricator.cockroachdb.org/D156