remove explicit support for PHP5 SWIG bindings in the Autotools build system

remove explicit support for PHP5 SWIG bindings in the Autotools build system

Older versions of SWIG can generate bindings for a variety of PHP versions.
Periodically SWIG bumps the version of PHP targeted by the default flag,
`-php`:¹

  SWIG version       `-php` effect   PHP5 support?   PHP7 support?
  ================================================================
  <1.3.26            `-php4`         no              no
  ≥1.3.26, <1.3.37   `-php4`         yes             no
  ≥1.3.37, <3.0.11   `-php5`         yes             no
  ≥3.0.11, <4.0.0    `-php5`         yes             yes
  ≥4.0.0             `-php7`         yes             yes

This commit changes the behavior under the Autotools build system:

  SWIG version       old behavior     new behavior
  ================================================
  <1.3.26            target PHP4  ==  target PHP4
  ≥1.3.26, <1.3.37   target PHP5  →   target PHP4
  ≥1.3.37, <3.0.11   target PHP5  ==  target PHP5
  ≥3.0.11, <4.0.0    target PHP7  ==  target PHP7
  ≥4.0.0             target PHP7  ==  target PHP7

It seems reasonable to consider the first three rows of the above table
irrelevant. The last version of PHP5, 5.6, was EOLed on 2018-12-31,² so any
users in rows 1-3 of the table have been unsupported for some time. In practice,
only users in row 2 are affected by this change, who are using a version of SWIG
released prior to 2009-01-12.³

In future, it may make sense to drop build system support for the `-php` flag
altogether and *only* support `-php7`, given that using `-php` on an older
version of SWIG that defaults to PHP5 probably produces uncompilable code
anyway.

¹ http://www.swig.org/Doc4.0/Php.html
² https://www.php.net/eol.php
³ https://github.com/swig/swig/releases