handle cmake & ccache

Summary: one place missed CMAKE_CXX_COMPILER_ARG1 support,

this allows to call cmake as:
cmake -D CMAKE_CXX_COMPILER="ccache" -D CMAKE_CXX_COMPILER_ARG1="g++"

http://stackoverflow.com/questions/1815688/how-to-use-ccache-with-cmake
Closes https://github.com/facebook/hhvm/pull/3601

Reviewed By: @sgolemon

Differential Revision: D1523463