Add path for llvm of homebrew/versions
CMake: remove USE_RPATH and rename {BUILD => GENERATE}_DOXYGEN

There was no point in providing USE_RPATH when the user can just use the
standard CMake variable 'CMAKE_INSTALL_RPATH_USE_LINK_PATH' directly.
CMake: detect and use libc++ when needed

See https://github.com/Sarcasm/irony-mode/issues/113
Merge pull request #118 from tsukimizake/master

Add path for llvm of homebrew/versions