Revert the main logic change from commit 827da1119e (Makefiles: Make
build root targets "all", "clean" and "preinstall" recursive,
2019-05-17, v3.15.0-rc1~96^2~2) for the "all" and "preinstall" targets.

The commit cleaned up the Makefile generator to use the same logic for
the "all" target in the top-level directory as for subdirectories.  It
exposed a long-existing bug that caused the "all" target in a
subdirectory to include the "all" targets from sub-subdirectories even
if they are marked `EXCLUDE_FROM_ALL`.  The `Tests/SubDir` test should
fail but the problem is currently covered up by another bug introduced
by commit dc6888573d (Pass EXCLUDE_FROM_ALL from directory to targets,
2019-01-15, v3.14.0-rc1~83^2) that causes the "all" targets in
`EXCLUDE_FROM_ALL` subdirectories to be empty.

Revert the top-level "all" and "preinstall" targets to the old approach
to prepare to fix the latter bug without exposing the long-existing bug
at the top-level.  Leave the "clean" target in the new approach because
it does not honor `EXCLUDE_FROM_ALL` anyway.

Issue: #19753