* tests/lt_dladvise.at, tests/lt_dlexit.at,
tests/need_lib_prefix.at: Skip these tests when called from
`make distcheck DISTCHECK_CONFIGURE_FLAGS=--disable-ltdl-install'.
The tests try to link against the dummy installed libltdl.la
from .../_inst/lib/libltdl.la, which does not exist when
configured with --disable-ltdl-install.