mod_brotli: Update makefile to allow using Brotli library >= 0.6.0.

The 0.6.0 version has just been released [1], and it contains the
necessary API required for mod_brotli.

[1] https://github.com/google/brotli/releases/tag/v0.6.0

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1790852 13f79535-47bb-0310-9956-ffa450edef68