[CMake] Perform operations optionally according to argument given to make call

[Olivier Pierard]

Hi,

I would like to perform some operations (typically search for packages,
add include dir and library) only for a given argument to the make call,
not in all the cases.  Typically, if I call 'make gui', the search has
to be done.  I don't want the general include_directories polluted by
this particular case.

Thanks,

Olivier