include/buffer: make static_assert c++11-safe

[liewegas]

The variant with only one arg was added in c++17.

Signed-off-by: Sage Weil sage@redhat.com

[tchaikov]

not sure if test_envlibrados_for_rocksdb.sh is able to verify it. as rocksdb compiles using -std=c++11.

[liewegas]

test_librados_build.sh is failing, http://pulpito.ceph.com/sage-2018-10-30_05:25:17-rados-wip-sage2-testing-2018-10-29-2124-distro-basic-smithi/3203167

[rzarzynski]

@liewegas, @tchaikov: apologizes for late review.

I'm afraid we might have a more severe problem as the failure seems to be triggered rather by a GCC version than just -std=c++11. On my machine, with GCC 7.2.0, passing -std=c+11 doesn't fail the compilation on the one-param-taking static_assert till providing -pedantic -Werror as well. And it looks we use a few C++1{4,7} constructs in our public headers (e.g. std::enable_if_t from C++14 in encoding.h and denc.h).

[liewegas]

Pretty sure neither enocding.h or denc.h appear in librados-devel (or anything else we distribute). The separation between common and include does not mean or imply anything.