mod_brotli: Rewrite the autoconf script in a, hopefully, less convoluted way.

Explicitly handle different cases when we do a pkg-config lookup
(<nothing>, --with-brotli or --with-brotli=yes) or examine the path
provided by the user (--with-brotli=PATH).

This lays the groundwork to simplify the switch to the official Brotli
library (https://github.com/google/brotli), instead of expecting the
install layout of a third-party wrapper (https://github.com/bagder/libbrotli).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1771789 13f79535-47bb-0310-9956-ffa450edef68