Remove special CUSTOM_DATA_INSTALL_PATH for OSX

The special codepath for CUSTOM_DATA_INSTALL_PATH on OSX means the path is
generated multiple backslashes which cause problems later on in the compile.
Having the same codepath as everything else appears to resolve this problem.
Merge pull request #32 from goetzk/custom-data-install-path-fix

Remove special CUSTOM_DATA_INSTALL_PATH for OSX