Maint: .gitignore: add m4/ltargz.m4 added with newer libtool

Likely related libtool change:
http://git.savannah.gnu.org/cgit/libtool.git/commit/?id=cdb6ac2df44a89d729b3504b33aeae38dcde3357
which would mean this is relevant when building with libtool v2.4.4+.

Also make sure the whole m4-rooted files are absolute within the repo,
we don't expected such subpath to occur anywhere else.