Makefiles: Make build root targets "all", "clean" and "preinstall" recursive

In the "Unix Makefiles" generator, the subdirectory targets "all", "clean" and
"preinstall" in Makefile2 are recursive.  In the build root directory,
these targets aren't.  Instead they're are added separately and additional
dependencies are added on a per target basis.  This is inconsistent and it
complicates per directory commands, like a per directory clean command.

This patch makes the "all", "clean" and "preinstall" targets in Makefile2
in the build root directory recursive, using the same algorithm that is
already used for subdirectories.

Some side effects are:
- Makefile2 gets smaller and simpler
- The main "all", "clean" and "preinstall" targets have recursive dependencies,
  instead of flat (depth of 1) ones.