cmake: warn if libstdc++ older than 5.1.0 is used

cmake: warn if libstdc++ older than 5.1.0 is used

__GLIBCPP__ is used before 3.4.0, and gcc 4.8.0 and up is required to
build c++11 source, hence it's safe to check __GLIBCXX__. for the
versioning of libstdc++, see
https://gcc.gnu.org/onlinedocs/libstdc++/manual/abi.html#abi.versioning,
for the magic number of 20150422ul see
https://gcc.gnu.org/develop.html#timeline.
for the reason why we want to have this warning, see
https://github.com/ceph/ceph/pull/18755#issuecomment-342736554 .

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