Log Message:
Update devel/autoconf to 2.70

2.70 deprecates a lot of stuff, so expect more warnings, but generally things
seem to work fine, so updating to 2.70 shouldn't break much.

Also update mk/gnu-config/*, as per the comment in devel/autoconf/Makefile.
mk/gnu-config/missing is not actually part of autoconf, but of automake, which
I did not update - however, the file was quite out of date, so I took the
liberty to update that one with the latest automake.

The change log is too long to include in this commit, given how many years
there were between 2.69 and 2.70. Check the file ChangeLog after `make
extract`.