With the removal of all legacy code paths, obsconfig.h always exists and
the compile definition always gets set. As such, it's no longer
necessary to check for it.
As removing the definition itself could be seen as a breaking change,
this simply moves the definition to pc.in and cmake.in files for now to
preserve the value for plugins that might expect this to be set. We may
remove the definition entirely in a later release.
When a unique path is set up as the location for user settings,
profiles, or scene collections, migrating the files from one computer
to another will lead to confusing error messages as the original paths
might not exist on the new machine.
The default directories for configuration files are already created by
this point, so using those paths as a fallback should enable OBS Studio
to also create the corresponding fallback settings files.
