slang: multiple fixes

Add a gentoo patch to fix the wicked slsh link line that ended in the
bizarre prefix/exec_prefix/DESTDIR trickery.
Also fixes:
http://autobuild.buildroot.net/results/c3f/c3fb5337a4bdf87baead64106427c4929241c58d/



Also be explicit with enabling/disabling libpng, pcre, readline and zlib
support since they get picked up from the host if header files are
around because of the problem known as "absolute paths" when trying to
autodetect.

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