treat empty PLIST_VARS.x as undefined?

[Tobias Nygren <tnn%NetBSD.org@localhost>]

Hi,

I had to fix a PLIST issue in devel/spdlog and tried to use
the nice and idiomatic OPSYSVARS approach.
This didn't work however:

OPSYSVARS+=		PLIST.dylib PLIST.notdylib
PLIST_VARS+=		dylib notdylib
PLIST.dylib.Darwin=	yes
PLIST.dylib.*=
PLIST.nodylib.Darwin=
PLIST.notdylib.*=	yes

The problem is empty string is treated like yes by plist.mk.
I guess that is why everyone is doing "if ${OPSYS}" kludges for
these. Doing like this works:

OPSYSVARS+=		DYLIB NOTDYLIB
PLIST_SUBST+=		PLIST.dylib=${DYLIB} PLIST.notdylib=${NOTDYLIB}
DYLIB.Darwin=
DYLIB.*=		@comment
NOTDYLIB.Darwin=	@comment
NOTDYLIB.*=

But it's a hack and not very clean.
Can someone suggest a fix to better way or an mk/ fix to support this
use-case properly?

Side-note: I think it is warranted to make this substitutions
pkgsrc-global, at least for packages that use CMake. There are
quite a few instances of them now.

Kind regards,
-Tobias