fix compilation with distcc and other compiler wrappers

fix compilation with distcc and other compiler wrappers

When building with ccache, distcc, and other compiler wrappers (such
as STLFilt):

CC='ccache gcc' CXX='ccache g++' cmake /path/to/ceph
make

python modules fail to compile since distutils try to execute the
wrapper itself without specifying the actual compiler.
Although cmake has a special magic switch for compiling with ccache
(cmake -DWITH_CCACHE=ON) other tools (distcc) are not supported, and
specifying the compiler as

CC=/whatever/compiler/is

used to work for decades, and it's a good idea to keep it working

Signed-off-by: Alexey Sheplyakov <asheplyakov@mirantis.com>
Signed-off-by: Kefu Chai <kchai@redhat.com>