This ensures all .c files which include a header file directly or
indirectly are rebuilt whenever the header file is changed.

Closes GH-6693.

Currently mail related tests are split for *nix and Windows (if there
are even Windows versions).  The basic difference is that the *nix
variants set the INI directive sendmail_path to just write the email to
disk, while the Windows tests use ext/imap.  The latter tests are way
more verbose, and such duplicated tests are generally a pain point.
Furthermore, the Windows tests are much slower, and could not be run
without ext/imap being available.

We therefore introduce a small fakemail application, which basically
works like `tee <path> >/dev/null`, and which will be shipped with the
Windows tests packs.  fakemail.exe would also need to be added to the
PHP binary SDK, so these tests could be run during developments.

To cater to the remaining differences, we also introduce support for
`{MAIL:<path>}` placeholders in the INI sections to run-tests.php.  How
to use this can be seen in mail_basic.phpt, which is currently the only
modified test case, because these tests are yet supposed to fail on
Windows, due to the missing fakemail.exe in the PHP SDK.

 * Change a number of "resource used as offset" notices to warnings,
   which were previously missed.
 * Throw the "resource used as offset" warning for isset() as well.
 * Make array_key_exists() behavior with regard to different key
   types consistent with isset() and normal array accesses. All key
   types now use the usual coercions and array/object keys throw
   TypeError.

Closes GH-4887.

The major version number is no longer included.