CMake: don't enable CXX unless building tests/benchmarks

[jameshilliard]

We only need CXX support when building tests/benchmarks.

Fixes:

CMake Error at CMakeLists.txt:4 (PROJECT):
  No CMAKE_CXX_COMPILER could be found.

  Tell CMake where to find the compiler by setting either the environment
  variable "CXX" or the CMake cache entry CMAKE_CXX_COMPILER to the full path
  to the compiler, or to the compiler name if it is in the PATH.

[jameshilliard]

@Maratyszcza This look good to merge?