guile: handle readline support explicitly

Otherwise the host readline (+ncurses) development files can poison the
build causing breakage. Fixes:
http://autobuild.buildroot.net/results/d0a/d0a8ad9d3f19592e06df36408d6af745f7908165/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>