Don’t distribute tests/ac*.at.

Don’t distribute tests/ac*.at.

tests/ac*.at are generated files containing basic test cases for all
the public AC_* macros that can be invoked without arguments.  They’re
generated using a simple awk script, and we already require awk at
build time because of automake, so there is no reason to ship them in
the tarball.  If we don’t ship them in the tarball, we can simplify
the logic in tests/local.mk, and avoid writing these files to the
source directory in a split build.

This should fix a problem with split builds using Solaris’ dmake
(see bug #110289).

tests/mktests.sh probably doesn’t work right if any of its argument
paths have spaces in their names, but that’s a separate issue.

* tests/local.mk (tests/mktests.stamp): Don’t change directory or
  rewrite the contents of $(AUTOCONF_FILES).
  (TESTSUITE_GENERATED_AT): Remove $(srcdir) prefix.
  Add tests/acerlang.at (accidentally omitted).
  (CLEANFILES): Add $(TESTSUITE_GENERATED_AT), mktests.stamp and mktests.tmp.
  (MAINTAINERCLEANFILES): Don’t set.
  (EXTRA_DIST): Include only the hand-written .at files, $(TESTSUITE_HAND_AT).

* configure.ac: Run AC_PROG_AWK, if for some reason AM_INIT_AUTOMAKE
  hasn’t done it for us.
* tests/mktests.sh: Use $AWK if set in environment.  Shell script linting.