Fix GCC atomic implementation in C++03
Pull in r250802 from upstream libc++ trunk (by Eric Fiselier):
Detect relaxed constexpr rules for gcc versions
Pull in r255585 from upstream libc++ trunk (by Eric Fiselier):
Fix various GCC mis-configurations for newer versions.
This patch goes through and enables C++11 and C++14 features for newer GCC's.
The main changes are:
1. Turn on variable templates. (Uses __cpp_variable_templates)
2. Assert atomic<Tp> is trivially copyable (Uses _GNUC_VER >= 501).
3. Turn on trailing return support for GCC. (Uses _GNUC_VER >= 404)
4. XFAIL void_t test for GCC 5.1 and 5.2. Fixed in GCC 6.
Together, these should fix building clang 3.8.0 as part of building
world with recent versions of gcc (e.g. the devel/*-xtoolchain-gcc
ports).
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