Release candidate

[Akim Demaille]

Please people, test it: the current state of Autoconf is our release
candidate.  There is one issue that must be fixed before releasing,
namely the test `compile.at:AC_PROG_CPP via CC' is not robust to
broken cc for not using AC_PROG_CC (but it concerns only the test
suite itself).

Other than that, this week should be week W.