cmake: Allow customization of the doc installation dir.
cmake: Allow customization of the doc installation dir.

Sample usage:

  cmake -DDOC_INSTALL_DIR=doc/apitrace-x.y.z ...

Fixes https://github.com/apitrace/apitrace/issues/411

apitrace/apitrace,apitrace/apitrace,schulmar/apitrace,apitrace/apitrace,apitrace/apitrace,trtt/apitrace,schulmar/apitrace,schulmar/apitrace,trtt/apitrace,schulmar/apitrace,trtt/apitrace,trtt/apitrace,trtt/apitrace,schulmar/apitrace