fix trac 9544: gcc 4.9.0 -std=c++1y breakage

This is GCC bug PR C++/59681 but the Boost fix is trivial and the GCC
one is (probably) not.

See: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59681
Merge branch 'fix/9544' of https://github.com/pabigot/boost-regex into develop
Merge branch 'develop'