cmake: Abort with an helpful message if an user tries to invoke cmake in gui subdir.
cmake: Abort with an helpful message if an user tries to invoke cmake in gui subdir.

More than one once I received reports of failures to build qapitrace because the
user presumed cmake should be invoked in the gui subdirectory.

Nip that practice in the bud.

schulmar/apitrace,tuanthng/apitrace,tuanthng/apitrace,trtt/apitrace,EoD/apitrace,apitrace/apitrace,trtt/apitrace,tuanthng/apitrace,EoD/apitrace,trtt/apitrace,apitrace/apitrace,schulmar/apitrace,tuanthng/apitrace,trtt/apitrace,schulmar/apitrace,trtt/apitrace,EoD/apitrace,schulmar/apitrace,schulmar/apitrace,EoD/apitrace,EoD/apitrace,apitrace/apitrace,apitrace/apitrace,tuanthng/apitrace