These were accepted in https://github.com/alexcrichton/gcc-rs/pull/212, but never actually worked and weren't being tested. This fixes the logic errors that prevented the new style from working, and also updates the tests to use the new style.
It allows one to add a flag to the compiler on the condition that the
compiler supports it.
We check if the compiler supports the flag by trying to compile a
simple executable. If there is some output on stderr we consider that
the compiler failed and the flag is not supported.
This adds two configuration options to optionally enable or disable
the `-shared` and `-static` flags. If omitted, musl targets enable
`-static` by default.