cmake: use imported GDAL target and clean up dependencies

[nilason]

• Use CMake's FindGDAL.cmake and use imported GDAL::GDAL target.
• Include GDAL include dir with public scope to grass_vector, grass_interpfl and grass_gproj targets enabling the use of those libraries as dependencies without adding GDAL::GDAL as dependency.
• Remove dependency of GDAL::GDAL from targets which do not need it.

[echoix]

I like this! It goes in the same thoughts as what I described here to be able to configure on windows last week. #5192 (comment)

The (draft) changes I made are here: echoix#379

I don't feel qualified to judge for the other changes, appart from the part of removing the custom FindGDAL and using the imported targets GDAL::GDAL; that looks fine to me.

[echoix]

One difference I see from their docs and here, is the find_package call that doesn't specify to use "CONFIG" mode. Does that mean it will use cmake's old FindGDAL instead of using config mode?

[nilason]

One difference I see from their docs and here, is the find_package call that doesn't specify to use "CONFIG" mode. Does that mean it will use cmake's old FindGDAL instead of using config mode?

It will use CMake's (currently 3.16+) FindGDAL. Bear in mind we still support pre-cmake builds of GDAL (<3.5) too.