Summary:
We have a global `--install-prefix` argument that can be used to set
the prefix for all projects, but that is only suitable if you are running with
sufficient privileges to install each of the deps to that location during the
build.  Cmake dependency resolution won't work from the build directory in that
situation; it can only see the final installed location and it will error out
if those files are not present, or link against the currently installed version
instead of the version we just built; not great!

This commit adds a project specific `--project-install-prefix` that can be used
on just the leaf project in a set of deps.  That sidesteps the dependency
concern because only the last stage is built in that mode.  This option
can technically be applied to an arbitrary set of projects, but in light
of the above, in practice it only makes sense to use it for the final
cmake project.  Only the CMakeBuilder respects this option.

In the watchman repo, this commit adjusts the autogen.sh script to allow
specifying the installation prefix; it defaults to `/usr/local` as you
might expect.

refs: https://github.com/facebook/watchman/issues/760

Reviewed By: yfeldblum

Differential Revision: D20674439

fbshipit-source-id: 52799dbd47f3c295e2d6469ee2b74cedeaa20138