testsuite: cope when some installed libltdl.la has been removed.

testsuite: cope when some installed libltdl.la has been removed.

* tests/old-m4-iface.at (AC_WITH_LTDL): Pass --with-included-ltdl
to configure, to ensure we do not accidentally link against a
previously installed libltdl.so where the corresponding .la
file has been removed, thus we do not add a run path, and the
runtime linker then fails to find libltdl.so.7.
* tests/configure-iface.at (--with-ltdl-include/lib): Remove -L
flags from LDFLAGS to avoid picking up an installed libltdl
outside of the default locations.
* THANKS: Update.
Prompted by report from Patrice Fromy.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>