* libltdl/configure.in: reintroduce code to detect native library

* libltdl/configure.in: reintroduce code to detect native library
extension
* libltdl/ltdl.c (lt_dlopen): if a .la file name is given, that's
all.  Otherwise, lt_dlopen now tries to append .la first.  If this
fails, it tries the given filename, and, at last, tries to append
the native library extension to the given filename.
* doc/libtool.texi (lt_dlopen): explain new lookup strategy