manual: give example where _INSTALL_TARGET = NO

The tutorial for autotools-package and cmake-package currently gives
the bad example of setting _INSTALL_TARGET to YES, which is the default.
So change this into an example with _INSTALL_TARGET = NO, and explain in
which case this is relevant.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>