ImageJ.c: run ij.ImageJ when not patching IJ1

When using the --dont-patch-ij1 flag, Fiji and ImageJ2 modes cannot
work. So, in the absence of any explicitly specified main class,
let's run ij.ImageJ by default when that flag is given.
Merge pull request #7 from imagej/dont-patch-ij1

ImageJ.c: run ij.ImageJ when not patching IJ1