devel/gwenhywfar-fox16: Fix build with llvm16

theme.cpp:20:3: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
  register FXint rr,gg,bb,dr,dg,db,r1,g1,b1,r2,g2,b2,yl,yh,yy,dy,n,t;

Reported by:	pkg-fallout
MFH:		2023Q3

devel/gwenhywfar/Makefile

@@ -64,6 +64,7 @@ INSTALL_WRKSRC=	${WRKSRC}/gui/${SLAVEPORT}
 LIB_DEPENDS+=	libFOX-1.6.so:x11-toolkits/fox16
 CONFIGURE_ARGS+=--with-fox-includes=${LOCALBASE}/include/fox-1.6 \
 		--with-fox-libs=${LOCALBASE}/lib
+CXXFLAGS+=	-Dregister=
 .  elif ${SLAVEPORT}=="gtk2"
 LIB_DEPENDS+=	libfreetype.so:print/freetype2 \
 		libharfbuzz.so:print/harfbuzz \