Instead of writing warning messages to `stderr`, we employ PHP's error
handling to raise `E_WARNING` even for the single case where
`bc_rt_error()` has been called, since that did not actually error out.
We choose to call `php_error_docref()` directly in libbcmath, since
there is no upstream, and since other PHP core functionality is already
used in our bundled libbcmath. Accordingly, we remove `rt.c` so it will
not be accidentally used in the future.
Besides adapting a few existing tests, we add new tests so that the
warnings are tested at least once. We also get rid of the Windows
specific tests, since the warning behavior is now supposed to be
platform-agnostic.
