Fix for executing AutoRegen.sh. Revert a part of r187209.

Since r187209, which modified ltdl.m4, I was unable to execute
AutoRegen.sh, getting:

  ../configure:10779: error: possibly undefined macro: AC_LTDL_FUNC_ARGZ

This commit re-adds AC_LTDL_FUNC_ARGZ to ltdl.m4, as a quick fix. For me, this
corresponds to the configure file currently checked in.

(However, the ltdl library seems to be unused since r74924 in 2009,
except for the use of the LTDL_SHLIB_EXT macro in
bugpoint(?). Therefore, the right solution seems to try to get rid of
the local ltdl.m4 file, specified by autoconf/README.TXT.)