graphics/dcraw: Clarify LICENSE comment

The other options besides GPL2 are not suitable for pkgsrc.

graphics/dcraw/Makefile

@@ -1,4 +1,4 @@
-# $NetBSD: Makefile,v 1.48 2024/01/26 15:00:39 gdt Exp $
+# $NetBSD: Makefile,v 1.49 2024/01/26 15:58:44 gdt Exp $
 #
 # Changes can be found in the RCS file at
 # http://www.cybercom.net/~dcoffin/dcraw/RCS/dcraw.c,v
@@ -10,7 +10,8 @@ MASTER_SITES=	https://www.dechifro.org/dcraw/archive/
 MAINTAINER=	[email protected]
 HOMEPAGE=	https://www.dechifro.org/dcraw/
 COMMENT=	Raw digital camera decoder
-# Multiple options in the header; choose GPL2.
+# Multiple options in the header; choose GPL2 as the only
+# straightforward method suitable for pkgsrc.
 LICENSE=	gnu-gpl-v2
 
 WRKSRC=			${WRKDIR}/${PKGBASE}