Update OpenSSL.podspec

Bug fix: Build fails if Xcode is in unexpected location. 

Updated to use variable path instead of using hardcoded path to gcc in Xcode app directory.

Script expects Xcode to be located in Applications/Xcode.app. Some development environments require multiple versions of Xcode, which requires the app to use a non-standard path, e.g. Applications/Xcode5.1.1.app.