doc/man/Makefile: Mimic mandb(5) path hierarchy

The following allows developers to read locally generated man pages by
using the `pre-inst-env' script. For example:

  $ ./pre-inst-env man lttng-add-context

will open the `lttng-add-context.1' man pages in the build directory
under which the `pre-inst-env' was generated.

This is done by:

  1. Simlinking `build/doc/man{1,3,7,8}' to `build/doc/man'

  2. Adding MANPATH to `pre-inst-env'

The symlinking part is a hack to force `man' to use our current doc
layout, doing a less invasive change than would be required otherwise

Change-Id: I2ea1af779f237fe1808a1d44d4f3b1c3a8535e2d
Signed-off-by: Olivier Dion <odion@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>