cmake: regroup CMAKE_CXX_FLAGS and CMAKE_C_FLAGS

cmake: regroup CMAKE_CXX_FLAGS and CMAKE_C_FLAGS

- some options are only meaningful for C++, so move them into
  CMAKE_CXX_FLAGS.
- regroup CMAKE_CXX_FLAGS and CMAKE_C_FLAGS, and remove the duplicated
  options.
- do not reset CMAKE_CXX_FLAGS with ${CMAKE_C_FLAGS}, instead, always
  append ${CMAKE_C_FLAGS} to existing ${CMAKE_CXX_FLAGS}. so we don't
  clobber CMAKE_CXX_FLAGS set by command line.

Signed-off-by: Kefu Chai <kchai@redhat.com>