add_dependencies: Distinguish target v. file dependencies in error (#14050)

When called with a non-existent LHS target name the user may be trying
to add file-level dependencies.  Clarify the error message to explain
the difference between target-level and file-level dependencies.  Point
the reader at the commands and options needed for the latter.
automoc: Use a pre-build event in VS >= 7

In VS IDE generators add a pre-build event to perform automoc instead of
using a separate custom target.  This reduces the number of targets in the
.sln that need to be loaded by the IDE.

This also works around a VS 11 bug as discussed in issue 13900.

Suggested-by: Hauke Heibel <hauke.heibel@gmail.com>
Merge topic 'clarify-add_dependencies-error'

de13d68 add_dependencies: Distinguish target v. file dependencies in error (#14050)
Merge topic 'automoc-vs11-workaround'

20c99b1 automoc: Use a pre-build event in VS >= 7