cmake: doc related fixes
To reproduce
When building and installing dbus from the master branch with cmake, some differences to the previously used autotools build system were noticed.
Steps to reproduce the behavior:
- build and install dbus from the master branch using cmake
- compare the installed files with a build from the dbus 1.14 branch
Expected result
The problems mentioned should not be present.
Actual result
- the docbook-based documentation for dbus-faq, -specification, test-plan and tutorial are not available
- the link to the api documentation in the html overview file is broken if the api documentation is not created.
Edited by Ralf Habacker