Merge branch 'am-prog-mkdir-p-remove' into ng/master

Since we had to restate AM_PROG_MKDIR_P in master before the 1.13
release, we don't merge master directly in ng/master, to avoid spurious
merge conflicts (since AM_PROG_MKDIR_P should remain obsolete and
removed in Automake-NG).

We have thus create a temporary branch 'am-prog-mkdir-p-remove', based
off of master, which reverts the commit 'v1.12.4-158-gdf23daf', which
reinstated AM_PROG_MKDIR_P, and are merging that branch in 'ng/master'.

Once 1.13 is out and master becomes the starting point of Automake 1.14,
we'll merge the 'am-prog-mkdir-p-remove' branch back into master as well,
since we plan to remove AM_PROG_MKDIR_P once and for all in Automake 1.14.

* am-prog-mkdir-p-remove:
  AM_PROG_MKDIR_P: remove as obsolete, this time for good
  depcomp: avoid potential interferences from the environment
  depcomp: improve comments about the 'gcc' depmode
  compat: reinstate AM_PROG_MKDIR_P, for gettext

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>

diff --cc gen-testsuite-part
Simple merge

diff --cc lib/depcomp
Simple merge