MJ2 files are not using the API correctly.

mj2.{c,h} are outside the library but use internal functions and data structures.

Same problem with frames_to_mj2, mj2_to_frames, extract_j2k_from_mj2, wrap_j2k_in_mj2.

Before changing these files so that they use only external functions and structures, a decision has to be made so as to include or not the mj2.{c,h} files in libopenjpeg.

The presence of jp2.{h,c} in libopenjpeg tends to consider as consistent to include mj2.{c,h} also (or to exclude jp2.{c,h}, as in kakadu).