graphics/vkd3d: explicitly disable doxygen documentation

$ pkg install doxygen
$ make
[...]
!pdfTeX error: /usr/local/bin/pdflatex (file 8r.enc): cannot open encoding file
 ==> Fatal error occurred, no output PDF file produced!
mv: rename refman.pdf to ../vkd3d.pdf: No such file or directory
gmake[1]: *** [Makefile:2082: doc/vkd3d.pdf] Error 1

$ pkg provides 8r.enc
Name    : tex-dvipsk-5.995_2
Desc    : Convert a TeX DVI file to PostScript
Repo    : FreeBSD
Filename: usr/local/share/texmf-dist/fonts/enc/dvips/txfonts/tx8r.enc
          usr/local/share/texmf-dist/fonts/enc/dvips/context/q-8r.enc
          usr/local/share/texmf-dist/fonts/enc/dvips/base/8r.enc

graphics/vkd3d/Makefile

@@ -19,7 +19,7 @@ RUN_DEPENDS=	${LOCALBASE}/include/vulkan/vulkan.h:graphics/vulkan-headers
 USES=		gmake libtool localbase pkgconfig tar:xz
 USE_LDCONFIG=	yes
 GNU_CONFIGURE=	yes
-CONFIGURE_ARGS=	--with-spirv-tools
+CONFIGURE_ARGS=	--disable-doxygen-doc --with-spirv-tools
 TEST_TARGET=	check
 
 # XXX Move to a separate port (a la graphics/vulkan-headers) and maybe make