depcomp tests: avoid redundant runs for libtool tests

When running libtool-oriented tests, our 'depcomp.sh' script used
to run the same checks thrice: once after having run configure
with the '--disable-shared' option, once after having run it with
the '--enable-shared' option, and once by allowing configure to
automatically select which kind of library (or libraries) to build.

This was done to offer extra coverage for the less-common depmodes
(like "aix" of "hp2") where the name of the compiler-generated
depfiles can depend on whether libtool is in use *and* on which
kind of libraries libtool is building (static, shared, or both).
About this, see commit 'Release-1-10-24-ge89ec38' of 28-03-2007,
"* lib/depcomp (aix): Rewrite depmode in the spirit of the tru64
one".

But that extra coverage came at the price of *considerably* slowing
down the depcomp libtool-oriented tests, since for each of them the
tested source tree was configured and built *twelve* times (rather
than "just" four as it is for the non-libtool case).

So, to strike a balance between coverage and speed, we now run the
libtool-oriented tests with the three different configuration modes
*only* when it is possible that the depmode that will get selected
is one of those which can actually benefit from the extra coverage.

* tests/depcomp.sh: Adjust to only run the threefold libtool tests
when that can truly be potentially useful.  Add related explanatory
comments.

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