math/wxmaxima: fix build on powerpc

/usr/bin/c++ -DWXUSINGDLL -D_FILE_OFFSET_BITS=64 -D_THREAD_SAFE -DWXGTK -Isrc -isystem /usr/local/lib/wx/include/gtk3-unicode-3.0 -isystem /usr/local/include/wx-3.0 -O2 -pipe -fstack-protector-strong -fno-strict-aliasing -pthread -O2 -pipe -fstack-protector-strong -fno-strict-aliasing -std=gnu++11 -MD -MT src/CMakeFiles/wxmaxima.dir/SlideShowCell.cpp.o -MF src/CMakeFiles/wxmaxima.dir/SlideShowCell.cpp.o.d -o src/CMakeFiles/wxmaxima.dir/SlideShowCell.cpp.o -c /wrkdirs/usr/ports/math/wxmaxima/work/wxmaxima-Version-20.04.0/src/SlideShowCell.cpp
In file included from /wrkdirs/usr/ports/math/wxmaxima/work/wxmaxima-Version-20.04.0/src/SlideShowCell.cpp:32:
In file included from /wrkdirs/usr/ports/math/wxmaxima/work/wxmaxima-Version-20.04.0/src/SlideShowCell.h:33:
In file included from /wrkdirs/usr/ports/math/wxmaxima/work/wxmaxima-Version-20.04.0/src/Image.h:33:
src/Version.h:5:6: error: invalid token at start of a preprocessor expression