Dictionary System Includes, master branch (2022.04.25.)

Made atlas_add_dictionary(...) handle external include paths correctly, making them show up as SYSTEM include paths to the compiler.

This is a follow-up to !946 (merged). Unfortunately I missed that dictionaries were also not including externals quite correctly. 😦 I only noticed it while removing the work-arounds from atlas/athena, in preparations for including atlasexternals-2.0.131 in it...

This will also be necessary for ATEAM-826 to be fixed.