Added environment variables to disable PCH or override PCH directory.
Added environment variables to disable PCH or override PCH directory.

surround-io/oclgrind,mpflanzer/Oclgrind,mpflanzer/Oclgrind,mpflanzer/Oclgrind,bitfusionio/Oclgrind,bitfusionio/Oclgrind,surround-io/oclgrind,bitfusionio/Oclgrind,surround-io/oclgrind,surround-io/oclgrind,mpflanzer/Oclgrind,bitfusionio/Oclgrind