[avs_commons] Fixed doc target
[avs_commons] Fixed doc target

Summary: `make doc` didn't work because CMake generator expressions weren't evaluated

Test Plan: `./devconfig && make doc && firefox ./output/doc/html/index.html`

Reviewers: mkrawiec, kpanek, kfyatek

Reviewed By: mkrawiec, kfyatek

Differential Revision: https://phabricator.avsystem.com/D9572

AVSystem/avs_commons,AVSystem/avs_commons,AVSystem/avs_commons,AVSystem/avs_commons