Adding requirement for PCRE in configure.ac

[Rik]

2/19/11

John,

Could you take a look at requiring building with PCRE in configure.ac?  The
build experiences on Mac OS have demonstrated that POSIX regex are not
sufficient; moreover, most Octave coders were already implicitly writing
Perl-style regex.  I just checked in a changeset which updates the
documentation and all of the regexp instances in the codebase to use PCRE
style expressions (http://hg.savannah.gnu.org/hgweb/octave/rev/e4dbfe3019b1)

Cheers,
Rik