minor, provide means to specify a custom m4 file to be injected in the gen process
chore, provide an example custom defs file to implement custom allocators
Merge branch 'feat/custom-allocator'

* feat/custom-allocator:
  chore, provide an example custom defs file to implement custom allocators
  minor, provide means to specify a custom m4 file to be injected in the gen process
fix, PEBKAC, in tarball builds ignore scmver_03.clit regression test
hygiene, do not use subshells, use command groups instead