mod_brotli: Update makefiles to use the library layout of the official

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.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1771827 13f79535-47bb-0310-9956-ffa450edef68