games/ponscripter-sekai: unbreak on powerpc64

g++8 -O2 -pipe -Wl,-rpath=/usr/local/lib/gcc8 -Wl,-rpath=/usr/local/lib/gcc8 -c -Wall -DUSE_PPC_GFX -I/usr/local/include/SDL2 -I/usr/local/include -D_REENTRANT -D_THREAD_SAFE -I/usr/local/include -I/usr/local/include/smpeg2 -I/usr/local/include/SDL2 -I/usr/local/include -D_REENTRANT -D_THREAD_SAFE -I/usr/local/include/freetype2 -DLINUX -DUSE_OGG_VORBIS -DCONST_ICONV graphics_altivec.cpp
In file included from graphics_altivec.cpp:29:
/usr/local/lib/gcc8/gcc/powerpc64-portbld-freebsd13.0/8.3.0/include/altivec.h:34:2: error: #error Use the "-maltivec" flag to enable PowerPC AltiVec support
#error Use the "-maltivec" flag to enable PowerPC AltiVec support

^~~~~

PR: 236351
Reported by: Piotr Kubaj, pkg-fallout