Directory-level rules in `CMakeFiles/Makefile2` were previously
previously written by each directory's local generator using its own
decision for using relative or absolute paths.

Since commit d33b12d8 (Add support for build tree symlink inside
source tree, 2022-02-25, v3.24.0-rc1~583^2), each local generator
explicitly models the relationship between its source and build paths,
and uses this to determine when it is safe to use relative paths.
Because `add_subdirectory` supports arbitrary placement of the source
and build directories, different local generators may have different
relationships between their source and build paths.  This can cause
disagreement among rules written to `CMakeFiles/Makefile2`.

Restore consistency by always using the root local generator to write
rules to `CMakeFiles/Makefile2`.  Relative paths should always be
expressed w.r.t. the top-level build directory since that is the working
directory in which the `...