Merge r1761824, r1771789, r1771827, r1779111 from trunk:

mod_brotli: Unbreak building other filter modules without libbrotlienc.

Don't add -lbrotlienc to the MOD_LDFLAGS unconditionally.  And, when
adding it, use MOD_BROTLI_LDADD to avoid linking all filter modules
against this library.


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).


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

With the recent update (https://github.com/google/brotli/pull/464), the
official repository now produces pkg-config metadata files (brotli.pc),
and has a fixed library layout (libbrotlicommon/libbrotlienc/libbrotlidec)
on both Windows and Linux.  Expect this layout in the makefiles, and take
advantage of the pkg-config metadata, if it's available.


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

There's a typo in the upstream commit [1] (s/brotlicommon/libbrotlicommon)
that currently produces an unusable libbrotlienc.pc file, but hopefully
this is going to be fixed there.

[1] https://github.com/google/brotli/commit/fe9f9a9#diff-af3b638bc2a3e6c650974192a53c7291R409

Submitted by: kotkov
Reviewed by: kotkov, covener, jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1793464 13f79535-47bb-0310-9956-ffa450edef68