Our official builds use clang-3.4 (for macOS) and clang-3.8 for Linux.
Linux uses libstdc++-4.9. Make sure these two compilers are being tested
and that libstdc++-4.9 is being used for STL.
Add a macOS builder to get coverage there. This will use a newer
clang than we actually use, but as opposed to the linux builder will
test the platform-specific bits.
Finally, use the latest GCC and clang versions from a bionic image, so
we have some coverage of a popular distribution.
David Anderson
ff018a9a5d