Fixing mispellig of PACKAGE_ABS_DIR_UNDER_PROJECT_SOURCE_DIR (Trilinos #6218)

Brent noticied this as part of a problem with automated testing but this was
not causing any problems.
Allow <Project>_TRIBITS_DIR to be provided as a relative path (Triinos #6218)

This allows a little easier setup for TriBITS/Trilinos co-development by just
having to set:

   -D<Project>_TRIBITS_DIR:STRING=TriBITS

and not the full path.  Note the warning about forgetting to use 'STRING'
above.