>(It works fine with make from NetBSD 1.6.1_STABLE under NetBSD.)

Sure?  I just tried:

/NetBSD/tools/1.6.X/NetBSD-1.6.1_STABLE-i386/bin/nbmake

build from sources as of Aug 9, and got the same result:

nbmake: "/homes/sjg/make-tests/empty" line 2: Malformed conditional (!empty(MANINSTALL:Mcatinstall))
nbmake: "/homes/sjg/make-tests/empty" line 4: if-less endif
nbmake: "/homes/sjg/make-tests/empty" line 4: Need an operator
nbmake: Fatal errors encountered -- cannot continue

Note that if MANINSTALL is defined, its ok (same with -current make etc).
That said, empty() shouldn't vomit if a variable is undefined - it should
count as "empty".

--sjg