With the Makefile generator one can use `cd $subdir; make install` to build and
install targets associated with a given subdirectory.  This is not possible to
do with the Ninja generator since there is only one `build.ninja` file at the
top of the build tree.  However, we can approximate it by allowing one to run
`ninja $subdir/install` at the top of the tree to build the targets in the
corresponding subdirectory and install them.

This also makes sense for `test`, `package`, and other GLOBAL_TARGET targets.
It was already done for `all` by commit v3.6.0-rc1~240^2~2 (Ninja: Add
`$subdir/all` targets, 2016-03-11).