I hate the thing too even though I made it, and rather just get rid of
it. But we can't do that yet. In the meantime, this brings us more
inline with autoconf and will make it slightly easier for me to write a
pkg-config wrapper, which we need.
We don’t need to set -stdlib=libstdc++. This only works on Clang so it
is not good to set it globally. In addition, Clang knows to use
libstdc++ on Linux by default if no stdlib is set:
324f918438/lib/Driver/ToolChains/Linux.cpp (L456)
It’s a good policy to just leave off stdlib for now.
Fixes#29877.
