Reorganize clang config to avoid macro redefinitions and fix GCC-4.8, 4.9, and 5.0 detection.
Enable <thread> for gcc-4.8 and later.
Disable thread support headers depending on values of internal libstdc++ macros.
Rework Intel config to make it both more accurate in C++11 mode (hopefully!) and easier to maintain.
Disable __int128 support when the compiler is really CUDA.
See https://svn.boost.org/trac/boost/ticket/10418.
Merge branch 'develop' of https://github.com/boostorg/config into develop
Add helper macro - BOOST_ATTRIBUTE_UNUSED - to suppress unused type/variable warnings.
Doc regen.
Merge upstream branch 'origin/develop' into proposal/no_cxx14_macros

Signed-off-by: Kohei Takahashi <flast@flast.jp>