graphics/dspdfviewer: simplify logic for -Wno-error=extra-semi-stmt

I first bogusly added OSVERSION < 1300000 at the end, but in hindsight
and from looking at svnweb the version logic after 11.3 EOL is
v >= 1300014 || (v in 1104000 .. 1200000) || v >= 1201000
==
(v in 1104000 .. 1200000) || v >= 1201000
==
(v >= 1200000 && v < 1201000)

So the port should not relax the check on (early) 12.0, which we can as
well leave out.

While here, add a note about upstream #191 still being open.

Phabricator:	D26646

graphics/dspdfviewer/Makefile

@@ -42,11 +42,12 @@ CFLAGS+=	-Wno-error=undefined-func-template
 .endif
 
 .if ${OPSYS} == FreeBSD && ${OSVERSION} >= 1200039 && ${CHOSEN_COMPILER_TYPE} == clang
+# (rene) also related to issue #191 which is still open as of 2020-10-21
 CFLAGS+=	-Wno-error=zero-as-null-pointer-constant
 .endif
 
 # Fixes build failure for Clang 8.0.0
-.if ${OPSYS} == FreeBSD && (${OSVERSION} >= 1300014 || (${OSVERSION} >= 1103000 && ${OSVERSION} < 1200000) || ${OSVERSION} >= 1201000) && ${CHOSEN_COMPILER_TYPE} == clang
+.if ${OPSYS} == FreeBSD && && ${CHOSEN_COMPILER_TYPE} == clang
 CFLAGS+=	-Wno-error=extra-semi-stmt
 .endif