Automake misintegration and improving the DejaGnu testsuite

As fallout from fixing the bug that finally pushed me to contact the 
DejaGnu project, I have found that the DejaGnu testsuite itself does not 
properly integrate with Automake.  In particular, the tests driven by 
options.exp overwrite the site.exp file that Automake provides, and 
DejaGnu actually needs that file overwritten.  Fixing this bug is a 
straightforward but significant change that I initially proposed on 
bug-dejangu in 
<URL:http://lists.gnu.org/archive/html/bug-dejagnu/2018-10/msg00020.html> 
and am reproposing here for discussion:

While the command line srcdir takes precedence over a value from 
site.exp, there is a window between reading site.exp and reparsing the 
command line, and the search for the tool init file occurs in this window.

I suggest adding a search path for the tool init file and specifically 
supporting running DejaGnu at both top-level and in the testsuite 
directory. (Both of those can currently work, so breaking either would 
be bad.) I suggest searching for a tool init file on

    ${srcdir}/testsuite/lib/tool/${tool}.exp:${srcdir}/testsuite/lib/${tool}.exp:${srcdir}/lib/${tool}.exp

The last item on that path is the current behavior, on which some 
testsuites may depend. The first item is an enhancement proposal to 
allow tool init files to be kept separate from local testsuite library 
modules. Additional processing during the search would eliminate 
"testsuite/testsuite/" pairs if ${srcdir} ends with "testsuite".

Unless overridden, DejaGnu's srcdir should match Automake's srcdir.

There will need to be a different site.exp for the options tests 
eventually, but that is a different problem from DejaGnu misusing the 
Automake-provided site.exp. Also, where in the documentation should I 
insert a section for the configuration files DejaGnu searches? A new 
section in Reference?


-- Jacob