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.
bug75178.phpt fails on Windows, because the stderr output is not
interspersed with stdout output there, but rather is appended to the
end. The fix is analogous to bug72093.phpt.
Since `bcpowmod()` does not support non-integral operands, we have to
truncate these in addition to emitting a respective warning. We also
have to work with the truncated values in the following.
We recognize that the division by one to enforce the truncation is
actually overkill, but we stick with it for now, and shall tackle the
issue for PHP 7.3.
