Merge r1780441, r1780470, r1795928 from trunk:

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.

buildconf: add --help for the new behavior

buildconf's behavior is a little complicated (and was made more so with
apr-config mode), so add some self-documentation.

buildconf: check in APR[-util] m4 macros

Suggestion by wrowe. These files haven't changed in years; since we need
them anyway, check them in directly. buildconf will still copy over the
latest versions during setup, and SVN will tell us if there are any
changes.

Submitted by: jchampion
Reviewed by: jchampion, jim, ylavic


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1807761 13f79535-47bb-0310-9956-ffa450edef68