Fix the selection of the fast CRC32 path in Ceph Luminous.

Fix the selection of the fast CRC32 path in Ceph Luminous.

This ugly patch selectively adds the `-msse4.2` for compiling
`util/crc32c.cc` file when a building machine's toolchain had
successfully compiled a simple test program. The CXXFLAGS are
enriched even when `WITH_SSE42` is not set as, in Ceph, there
is no possibility to use the global switch because of lack of
proper run-time SSE4.2 check in other places IIUC.

Although the problem is fixed in `master` of RocksDB, the fix
cannot be backported because the CRC32-related stuff diverged
too much (`HAVE_POWER8`, `HAVE_PCLMUL`, `NO_THREEWAY_CRC32C`).

The patch strictly targets Ceph Luminous branch.

Fixes: http://tracker.ceph.com/issues/22534
Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>