Fix: static_assert unavailable with glibc < 2.16

Fix: static_assert unavailable with glibc < 2.16

gcc 4.8 introduces support for C11, and gcc 4.6 introduces support for
_Static_assert. Therefore, using _Static_assert when C11 is detected is
always OK.

However, using static_assert in C11 depends on glibc >= 2.16. Even
though the minimum version requirement for glibc is not documented in
the README.md file, make a best effort to keep compatibility with older
glibc.

Fixes: #1331
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I33b65b839ea5ecd0f710179a7ef6fb6f5fda2e17