try to use pyopencl -D to define preprocess definitions.

[john-hu]

kilik had found a way to define preprocessor definition at python, see #9 .

size = 100
clProg = cl.Program(ctx, fstr).build(options="-D GLOBAL_SIZE={0}".format(size))

Currently, we use code generation to put all preprocessor definitions. If possible, we should use build options to do so. I cannot make sure how long it can use or any limitation at that.