Correct m4 conditionals in curses.m4

Correct m4 conditionals in curses.m4

AS_IF does not invoke the test(1) shell builtin for us, so we must
take care to consistently use it ourself.

While here, sprinkle some missing double-quotes around variable
expansions in AS_IF statements in this file.

Submitted by Bastian Beischer.

FIXES 134414

Change-Id: Iccfe311011f17de6317cf64abdc58b0812b81b8c
Reviewed-on: https://gerrit.openafs.org/12738
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>