HOST should be used by tools only, not for host packages.

Other minor cleanups.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

mstch is an implementation of the the mustache templates using C++.
This package will build mstch static library that can be used by
other packages.

Signed-off-by: Amol Bhave <ambhave@fb.com>