remove ax_prefix_config_h.m4

[Peter]

It was introduced in r1319 and should in principle have been removed in r1321. Only reason it was not removed was to not break the build. A Makefile.in created on rev 1319 or 1320 will require access to the file, and if it is not there it will ^%$&$. Therefore, let's remove the macro when we think no one has that dependency, i.e, when everyone has issued make on rev 1321 (or later).

[Peter]

(In [1421]) fixes #371