cmake: Fix OSX post-build if not all targets are requested

The post-build actions for OSX were not conditional on the selected
Quassel variants, so add_custom_command would be added to an undefined
target in that case.

This bug has been there since literally forever; however, CMake < 3.0
silently ignored the custom commands in that case, and thus this
issue was not visible.

With cmake_minimum_required now set to 3.5, the new behavior is
enabled and causes an error at configure time. Fix this by adding
the missing conditions.