- Don't try to detect libc++abi. It doesn't actually work, so don't
try it.
- Correctly try to link the standard library.
This should fix the build on GNU/Linux systems with libc++abi.so
installed (including our Ubuntu CI!).
macOS ships with libc++abi, which doesn't provide the hooks required
for interop, so this was causing all of the tests to fail to link in
the Travis macOS test. We're now correctly detecting that it won't
work on macOS and disabling those tests.