Modernize, improve and/or extend test scripts `conf*.test'.

Modernize, improve and/or extend test scripts `conf*.test'.

* tests/confh5.test: Cosmetic changes.
* tests/conff.test: Likewise.
* tests/confdeps.test: Likewise.
* tests/conflnk.test: Likewise.
* tests/conflnk2.test: Likewise.
* tests/confsub.test: Likewise.
* tests/confvar.test: Likewise, and make grepping of Makefile.in
stricter.
* tests/confvar2.test: Likewise.
* tests/conflnk3.test: Cosmetic changes, and re-enable a temporarly
disabled test (which didn't work with autoconf <= 2.59, but now we
are requiring autoconf 2.62, so...)
* tests/conflnk4.test: Cosmetic changes, and extend existing tests
accordingly to "TODO" comments.
* tests/conff2.test: Make grepping of Automake's stderr stricter.
Add some comments explaining why we don't use the `configure.in'
stub preset be ./defs.
* tests/confh.test: Use the `configure.in' stub created by ./defs,
rather than writing one from scratch, and do not call AC_OUTPUT.
Enable `errexit' shell flag, and related changes.  Prefer diff over
cmp to compare text files. Prefer perl over sed to fetch the value
of $(DIST_COMMON) from Makefile.in.  Make grepping of the contents
of $(DIST_COMMON) stricter.
* tests/confh4.test: Use the `configure.in' stub created by ./defs,
rather than writing one from scratch.  Make grepping of Makefile.in
stricter.
* tests/confh5.test: Make grepping of `config.h' stricter.  Add a
comment.
* tests/configure.test: Avoid obsolescent constructs in generated
`configure.ac'.  Do not write `configure.in' two times.  Escape
literal dots in grep regular expressions.
* tests/confincl.test:  Enable `errexit' shell flag, and related
changes.  Prefer fgrep over grep.  Other cosmetic changes.
* tests/config.test: Renamed to ...
* tests/confh6.test: ... this.  Fix m4 quoting in `configure.in',
and make grepping of `config.h' and `config.h.in' stricter.
* tests/conf2.test: Renamed ...
* tests/confh7.test: ... to this.  Use the `configure.in' stub
created by ./defs, rather than writing one from scratch.  Try to
run the checks both with and without AC_PROG_CC and AC_OUTPUT in
`configure.in'.
* tests/Makefile.am (TESTS): Updated.