mod_brotli: Add initial implementation.

This new module supports dynamic Brotli (RFC 7932) compression.  Existing
mod_deflate installations can benefit from better compression ratio by
sending Brotli-compressed data to the clients that support it:

    SetOutputFilter BROTLI_COMPRESS;DEFLATE

The module features zero-copy processing, which is only possible with the
new API from the upcoming 1.0.x series of brotli [1].  The Linux makefile
works against libbrotli [2], as currently the core brotli repository doesn't
offer a way to build a library [3].  Apart from that, only the CMake build
is now supported.

[1] https://github.com/google/brotli
[2] https://github.com/bagder/libbrotli
[3] https://github.com/google/brotli/pull/332


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1761714 13f79535-47bb-0310-9956-ffa450edef68
