make: Rework headers generation and installation

Because we need to provide userspace API for plugins
and libcompel itself, here is a huge rework of make
engine.

The new target introduced

	make install-devel devel-include=<dir> devel-lib=<dir>

where <dir> are directories to put headers and plugins into.
If <dir> is omitted then "devel" directory will be created
in current working directory.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>