Use mutex to ensure printf output not mangled.
Use mutex to ensure printf output not mangled.

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