[CMake] Cleanup code which adds flags for external grids.

Move code from both if branches out of the if.
UseUG and AddALUGridFlags become more similar.
Fix indentig, make line breaks more consistent.
[CMake] Fix CMke warnings CMP0022 and CMP0038 for ALUGrid and UG.

Prevent dunegrid to link to itself. This happend when the flags for
ALUGrid or UG where added. This caused CMP0038 that will become an
error with CMake 3.0.0.
This fixed the annoying CMP0022 warnings about the link targets
interface, too.
For Alberta both issues remain yet un-fixed.