main/slang: fix detection of terminfo
ncurses-dev depends recursively on ncurses-terminfo-base, which has terminfo files in /etc/terminfo.
But slang only looks for terminfo files in /usr/{lib,share}/terminfo.
When that's not found, it determins it must link against termcap (adds
-ltermcap
to linker flags), which cannot be found and breaks the build.
Fix autoconf to look for terminfo in /etc/terminfo so to fix the build.