Create a backport branch with r1761824, r1771789, r1771827 and r1779111

Create a backport branch with r1761824, r1771789, r1771827 and r1779111
applied to 2.4.x.

These are the mod_brotli related Makefile changes that didn't make it
into the original backport proposal merged in r1791231.  The lack of these
changes causes a failing Unix build in my environment if mod_brotli is not
being built.  Another issue is that by default the CMakeLists.txt file
refers to invalid library filenames.

Shortlog of the changes:

    r1761824: Unbreak building other filter modules without libbrotlienc.

    r1771789: Rewrite the autoconf script in a, hopefully, less convoluted way.
    This lays the groundwork to simplify the switch to the official Brotli
    library.

    r1771827: Update makefiles to use the library layout of the official
    Brotli repository.

    r1779111: Update makefile to cope with the pkg-config layout change
    in https://github.com/google/brotli/commit/fe9f9a9

Also see https://lists.apache.org/thread.html/a83741642c3d20f383efdf2c427829b2f37cb81e98c0a1756a2c19ce@%3Cdev.httpd.apache.org%3E

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x-r1761824-group@1792805 13f79535-47bb-0310-9956-ffa450edef68