fix relative path in project export files

The earlier commit 6d07ec56bc83c3e1e085ec33cec089bc49595f07 already fixed
relative paths in export files for package export; this commit does the same
for project export files.

See Trilinos bug #6181.
Merge branch 'master' into export_include_dir
Merge pull request #7 from nschloe/export_include_dir

fix relative path in project export files