Make ProofGeneral.rcp work with Emacs >= 24.2. on OS X

See [this commit](https://github.com/dimitri/el-get/commit/9b09e5d4876a2536f05cb2459ea272549ba59f92#diff-8825f5390c141818253b79c59f5c496c).
the commit makes ProofGeneral.rcp work with Emacs >= 24.2. but, it doesn't support OS X.