Carry through build options from cmake command line

Summary: This makes it possible to carry through build options from the cmake
command-line, by, exporting the environment, eg.:
  CMAKE_C_FLAGS="${CFLAGS}" CMAKE_CXX_FLAGS="${CXXFLAGS}" cmake
in a complex build environment.
Closes https://github.com/facebook/hhvm/pull/3855

Reviewed By: @ptarjan

Differential Revision: D1580318