Makefile: Restore use of dependency scanning cache

Since commit v2.8.0~27 (Major optimization of C/C++ dependency scanning,
2009-09-23) our `VaildDeps` cache of `depend.internal` content is
supposed to avoid re-scanning dependencies of object files whose
dependencies have not changed.  However, this was broken by changes to
`cmDependsC::WriteDependencies` by commit v3.1.0-rc1~272^2~1 (cmDepends:
Refactor object file path conversion, 2014-07-22).  The object file path
written to `depend.internal` was changed to a relative path, but the
lookup in the `ValidDeps` cache of that information was not updated too.
Therefore the cache is not used.

Fix the object file path used for the `ValidDeps` lookup to restore the
original optimization.
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