Makefile: Fix regression in dependencies on relative includes

Since commit a13a5c948e (Replace use of CollapseCombinedPath with
CollapseFullPath, 2019-03-19, v3.15.0-rc1~361^2~1), one code path now
calls `CollapseFullPath` with a base path that may be relative.
Backport KWSys commit c6f8e24a3 (SystemTools: Fix CollapseFullPath with
relative base path, 2019-07-24) to handle such base paths.

This case occurs when a build tree is placed in a directory inside a
source tree such that CMake is willing to generate a relative path from
the build tree to the source tree.  Add a test covering this case.

Fixes: #19507

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