package/gawk: add optional readline dependency

The gawk debugger can optionally use readline for user input during
the debugging session (for the usual autocompletion and stuff).

Enable readline support when the readline package is enabled; let
./configure automatically find it. Forcibly disable readline support
when the readline package is disabled.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>