buildconf: allow configuration without APR sources

Previously we required copying the APR (and APR-util) source code into
srclib or some other location on disk in order to build httpd directly
from source. This is annoying if you're on a distribution that already
has the required files in its APR dev packages.

Practically speaking, if you're not building an official distribution
tarball, you only need the following files:

1) config.guess
2) config.sub
3) find_apr.m4
4) find_apu.m4
5) PrintPath

1 and 2 come from automake. 3 and 4 are included in some distributions'
(e.g. Debian's) development packages for APR/-util. That leaves
PrintPath, which has not changed meaningfully in over a decade and is
checked in completely here.

Passing an apr-config executable to buildconf's --with-apr option will
now enable a mode in which the above files (minus PrintPath) are copied
from their respective homes, removing the need for APR sources on disk.
Otherwise, if a source tree is passed, the current behavior is retained
and all of the above files are copied from APR directly.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/trunk-buildconf-noapr@1780441 13f79535-47bb-0310-9956-ffa450edef68