We no longer use POSIX style regexes, but rather PCRE regexes.  This
introduces a hard dependency on libpcre.

Test suite passes, but only lightly tested.

The regex functions are in its own source file to make it easy to
change the decision if somebody wants to use non-PCRE for some reason.

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4334 d4fa192b-c00b-0410-8231-f00ffab90ce4