Check for very old PCRE versions in installer and updater

RE_IPV6_ADD uses (?(-n)) ("relative reference condition"), and CSSMin uses
\K ("reset start of match"), which only work in PCRE 7.2 and later -- newer
versions than the PCRE 6.6 included with Red Hat Enterprise Linux 5 and its
derivatives (e.g. CentOS 5).

Because the WMF developers, in general, do not seem to support maintaining
compatibility with such old software versions, I have opted to add a check
to the MediaWiki installer for these versions of PCRE. Affected users are
directed to a MediaWiki.org page advising the use of a different PHP package
that uses the bundled PCRE version instead of the older system version.

For now, the minimum PCRE version is set to 7.2, the oldest version not
known to break MediaWiki core. Once PHP 5.3 support is dropped, we may be
able to require PCRE 8.12 (bundled with PHP 5.4.0) or later.

The existing check for mere existence of the PCRE functions is removed;
since PHP 5.3, it is impossible to compile PHP without PCRE support.

Bug: 58213
Change-Id: Icf3732b6f84eeb25990178ae8fe3bd0fe4cc833f