Support use of an external copy of the PCRE library:

* configure.in: Set abs_{builddir,srcdir} higher.  Add --with-pcre
flag; build against external PCRE library if used.

* Makefile.in (install-include): Don't install pcre headers any more.

* srclib/Makefile.in (SUBDIRS): Remove.

PR: 27550 (part two)
Submitted by: Andres Salomon <dilinger voxel.net>, Joe Orton


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@153400 13f79535-47bb-0310-9956-ffa450edef68