pkg-cmake: allow to build package in a subdirectory

For some cmake based packages, like GNURadio, it's forbidden to do the
compilation directly in the sources directory. This patch add a new
variable to specify, if needed, the name of a sub-directory used to compile.

[Thomas: put the documentation at the right place in the manual, not
in the middle of the <pkg>_CONF_OPTS description.]

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>