dejagnu: allow the package developer to extend site.exp

dejagnu: allow the package developer to extend site.exp

Fixes automake bug#7873.

* lib/am/dejagnu.am (site.exp): Depend from the files listed in
$(EXTRA_DEJAGNU_SITE_CONFIG), if any.  Append their contents to
the generated site.exp (still preserving user edits).
* doc/automake.texi (Dejagnu Tests): Update.
* tests/dejagnu-siteexp-append.test: New test.
* tests/dejagnu-siteexp-extend.test: Likewise.
* tests/dejagnu-siteexp-useredit.test: Likewise.ù
* NEWS: Update.
* tests/Makefile.am (TESTS): Update.

Suggestion by Rainer Orth.