Sanitize the installed config_vars.mk to remove references to the

Sanitize the installed config_vars.mk to remove references to the
build and source directories, and to remove variables which are
internal to the httpd build system:

* build/config_vars.sh.in: New file.

* Makefile.in (build/config_vars.out): New target.
(install-build): Install build/config_vars.out as config_vars.mk.

* configure.in: Add include path for srclib/pcre to INCLUDES rather
than CPPFLAGS, and use a topdir-relative not absolute path name.
Generate build/config_vars.sh.

Reviewed by: Paul Querna
PR: 20874, 28145

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@190392 13f79535-47bb-0310-9956-ffa450edef68