Allow installation to srcdir from external build dir.

Test that case, and the in-src build install to src case.
Merge topic 'install-prefix-in-interface' into next

17cb943b Allow installation to srcdir from external build dir.
Add missing files.
Merge topic 'install-prefix-in-interface' into next

f9ee2e14 Add missing files.
Give this test a more appropriate name.
Update comment.
Merge topic 'install-prefix-in-interface' into next

4701be76 Update comment.
5be950da Give this test a more appropriate name.