cmLocalGenerator: remove "virtual" where not used

This patch makes several functions of cmLocalGenerator which are marked
as virtual non-virtual, since they are not reimplemented anywhere.

Alex
export files: rewrite the code for checking required targets

Instead of generating a whole bunch of repeated if-statements
now a foreach()-loop is generated in the targets-file.
Also now a comment is inserted in the generated file if no
targets from other export sets are used, so if somebody looks
at the file he can see whether the information about missing
imported targets has been generated or not.

Alex
Merge topic 'ExportFileGenerator_GenerateNicerErrorCheckingCode' into next

11d0c66 export files: rewrite the code for checking required targets
Merge topic 'cmLocalGenerator_RemoveVirtuals' into next

f497dbc cmLocalGenerator: remove "virtual" where not used