httpdunit: a Check-based unit test suite

Add a unit test suite based on Check:

    https://libcheck.github.io/check/

The suite depends on the build system to automatically generate the code
stubs that call every test case.

httpdunit is automatically enabled in the build if configure is able to
find Check via pkg-config. At the moment pkg-config is the only official
(non-deprecated) way to build and link against Check with an autoconf
system, since platforms may distribute Check as a static library.

Note that Check is an LGPL'd library, so we can't distribute test
objects and binaries. Building and running the suite remains optional
and is not required to run the server.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/httpdunit@1796202 13f79535-47bb-0310-9956-ffa450edef68