change the DLL name to onig.dll when using MinGW

The DLL names were different:
  VC++:  onig.dll
  MinGW: libonig.dll
But the names should be the same, because they run on the same platform
and of cause they are binary compatible.
Merge branch 'onig-5.9.x-mod' into onig-5.9.x-mod_for_bregonig-v2