tests: avoid distcheck failure with --disable-ltdl-install.

[Ralf Wildenhues]

I've ran the various distcheck DISTCHECK_CONFIGURE_FLAGS=... tests
required for release on my system.  This is the only fixup I needed.

Pushed to master.

Cheers,
Ralf

    tests: avoid distcheck failure with --disable-ltdl-install.
    
    * tests/ltdl-libdir.at (libdir of installed modules): Skip if
    $LIBLTDL does not exist.

diff --git a/tests/ltdl-libdir.at b/tests/ltdl-libdir.at
index 7065c5b..01c9e70 100644
--- a/tests/ltdl-libdir.at
+++ b/tests/ltdl-libdir.at
@@ -93,6 +93,9 @@ main (int argc, const char *argv[])
 : ${LTDLINCL="-I$abs_top_srcdir/libltdl"}
 : ${LIBLTDL="$abs_builddir/../libltdl/libltdlc.la"}
 
+# Skip installcheck if --disable-ltdl-install was used.
+AT_CHECK([test -f "$LIBLTDL" || exit 77])
+
 CPPFLAGS="$LTDLINCL $CPPFLAGS"
 LDFLAGS="$LDFLAGS -no-undefined"