slang: fix linking issue

The installation location of the slang library
does not take into account the DESTDIR directory.
So SLANG_INST_LIB is initialized with -L/usr/lib/
and slang may be linked with host's libdl.so (if any)
Therefore, we have to pass correct installation paths.

Fixes:
http://autobuild.buildroot.net/results/df4/df435d9cebbdc84b3581ba258b618123570dde8b/



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>