benchmark

Travis-ci: fix clang+libc++ build (#783)

It broke because the libc++ is being built as part of *this*
build, with old gcc+libstdc++ (4.8?), but LLVM is preparing
to switch to C++14, and gcc+libstdc++ <5 are soft-deprecated.

Just the gcc update doesn't cut it, clang still uses old libstdc++.