Build libltdl recursively instead of as a sub-project

Building libltdl recursively now eliminates the second 'configure' run
inside the libltdl directory and requires the distribution of fewer
files and quiets compilation.

Also, if the 'configure' option '--with-included-ltdl' is not passed and
a system libltdl is found, the included libltdl directory will not even
be built.  Only when the included libltdl is needed will it be built.