build-sys: explicitly check for slang.h

build-sys: explicitly check for slang.h

The slang/slcurses.h contains

  #includes <slang.h>

but we don't use -I/usr/include/slang (and this is also missing in
slang.pc), it means that we have manually include the slang.h file
in our configure script.

Note this is Fedora-12, maybe some others distros have more usable
slang headers...

Signed-off-by: Karel Zak <kzak@redhat.com>