Set our default cxx and c flags to clang too

This code needs a bit of refactoring to share most of it between gcc and
clang
6 jobs for more_bugprone_checks in 11 minutes and 21 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #692164
build

00:10:32

passed #692169
build_android

00:02:22

passed #692165
build_clang_libcpp

00:05:04

passed #692168
build_clazy

00:03:47

passed #692167
build_mingw64_fedora

00:04:47

passed #692166
build_ubuntu_16_04

00:03:08