pg_config.h is supposed to be placed right besides libpq-fe.h, so we
should check the same paths.

Otherwise the test fails.

The php_stream_read() and php_stream_write() functions now return
an ssize_t value, with negative results indicating failure. Functions
like fread() and fwrite() will return false in that case.

As a special case, EWOULDBLOCK and EAGAIN on non-blocking streams
should not be regarded as error conditions, and be reported as
successful zero-length reads/writes instead. The handling of EINTR
remains unclear and is internally inconsistent (e.g. some code-paths
will automatically retry on EINTR, while some won't).

I'm landing this now to make sure the stream wrapper ops API changes
make it into 7.4 -- however, if the user-facing changes turn out to
be problematic we have the option of clamping negative returns to
zero in php_stream_read() and php_stream_write() to restore the
old behavior in a relatively non-intrusive manner.

These checks were once relevant for these extensions in PECL and PHP
versions without availability of the checked macros.

Closes GH-4405

Conditional checks were once used for backwards compatibility with
phpize from PHP versions that didn't have this macro call yet.

Closes GH-4376

RFC: https://wiki.php.net/rfc/tostring_exceptions

And convert some object to string conversion related recoverable
fatal errors into Error exceptions.

Improve exception safety of internal code performing string
conversions.