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 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.