tests: workaround for automatic linker determination and conditionals

tests: workaround for automatic linker determination and conditionals

See automake bug#11089.

Automake is not very smart in automatically determining the command
to be used to link a program whose source files' languages are
conditionally defined.  For example, an input like:

  if HAVE_CXX
  foo_SOURCES = more.c++
  else
  foo_SOURCES = less.c
  endif

will cause the build rules for 'foo' to *unconditionally* use the
C++ compiler for linking, even when the 'HAVE_CXX' conditional
evaluates to false (which might mean that no C++ compiler is
available).

This behaviour is not really correct, but it's easy enough to work
around, and it's only relevant for fringe use cases (at best).  So
let's just test that the workaround really works.

* tests/link_cond.test: New test.
* tests/list-of-tests.mk: Add it.
* THANKS: Update.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>