kconfig: search harder for curses library in check-lxdialog.sh

The check-lxdialog.sh script searches for "libFOO.so" which fails on OS X, due
to their special naming of libraries like "libfoo.dylib".  This patch turns
the curses lib search into extensible loops and adds dylib as a valid
extension.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>