* Use GNUInstallDirs instead of hardcoded paths
This change should be a no-op in the normal case and at the same time make it
easier to customise polybar builds on systems with special needs.
* Avoid creating /usr/share/doc/polybar/polybar/*
* Include GNUInstallDirs for the doc target itself
* cmake: Don't try to set CMAKE_INSTALL_* variables
Since we include GNUInstallDirs all these variables are already set
* cmake: Print install directories in summary
* fix(cmake): Make doc-only work like normal build
This is kind of a dirty hack to force CMAKE_INSTALL_DOCDIR to use
`polybar` as the project name when only polybar-doc is built.
Maybe it is wiser at some point to be able to do a doc only build (and
install) that can be done from the top level project. Then we would also
not need to include GNUInstallDirs here
