Modernize and improve test scripts `dist*.test'.

Modernize and improve test scripts `dist*.test'.

* tests/distcleancheck.test: Do not add useless `-e' option to
a $MAKE call.  Extend test by grepping stderr of make.
* tests/distcom2.test: Do not run the same test script on the
Makefile.in twice, but save its output in an intermediate file
instead.  Make grepping of DIST_COMMON definition stricter.
Display the content of more files, to ease debugging.  Add a
trailing `:' command.  Improved heading comments w.r.t. sister
test(s).
* tests/distcom6.test: Likewise, and avoid to uselessly run
autoconf.
* tests/distcom3.test: Ensure verbose printing of captured stdout
and stderr.  Make grepping of captured stderr stricter.  Also,
add trailing `:' command.
* tests/distcom4.test: Declare the target `test' in the generated
Makefile.am as `.PHONY'.  Display content of more files, to ease
debugging.  Add trailing `:' command.
* tests/distcom5.test: Likewise.  Also, factor out common sed
script in subroutine `extract_distcommon'.
* tests/distcom7.test: Prefer cat + here-doc over echo to write
test Makefile.am files.  Add a trailing `:' command.
* tests/distname.test: Prefer `gzip -d' over `gunzip'.  Move the
call to `set -e' earlier.  Be stricter and more verbose in the
checking of the generated tarball.
(configure.in): Use the stub provided by ./defs, instead of
writing it from scratch.  Avoid obsoleted constructs.  Remove
useless call to `AM_PROG_CC_C_O'.
* tests/distdir.test: Various minor improvements/normalizations.
* tests/distlinks.test: Likewise.