.omc should be invalidated when OMAKEPATH value changes

[ANogin]

I installed OCaml 4.07.1 via opam (under docker) and built a project using OMake. Then I installed OCaml 4.09.0 (in a different docker image, so the 4.07.1 was no longer there). I tried running omake on the same source directory again. I got:

*** omake error:
   File OMakeroot: line 9, characters 2-14
   can't open IR: /usr/local/opam/4.07.1/lib/omake/build/C.om
*** omake: polling for filesystem changes (OMakefiles only)

The "offending" line is just open build/C. The problem appears to be that it tries to follow the location cached in .omc during the previous build.

In my use case, at least the issue is fairly obvious, but if I still had the old omake around (and it was an older version with older standard library!), it would probably be silently using the old location rather than the current one.

[jhwoodyatt]

Workaround for this: omake --flush