Add M4 files to tarball

[clementperon]

If I understood well STACK_DIRECTION is mandatory to compile libsigsev

I try to compile without it and failed at

./handler-unix.c:196:29: error: no member named 'is_near_this' in 'struct vma_struct'
  196 |                       : vma.is_near_this (addr, &vma))
      |                         ~~~ ^
1 error generated.

My main issue was that in the tarball release even with the latest autotools version you can't properly build libsigsev without pulling gnulib.

Add the required M4 files to the repo