add the libdir to the ltdl lib search path

add the libdir to the ltdl lib search path

* libguile/Makefile.am (libpath.h): Fix pkgdatadir, pkglibdir, and
  pkgincludedir entries. Add a new define, SCM_LIB_DIR.

* libguile/dynl.c (sysdep_dynl_init): Add the libdir to the libltdl
  search path. Should fix
  http://thread.gmane.org/gmane.lisp.guile.bugs/4289/focus=4296 -- that
  is, it should allow guile to be invoked from whereever it is
  installed, without munging LTDL_LIBRARY_PATH or the like variables.

  There is a trick though -- during the build, we don't want to be
  looking in the $libdir for loadable modules. So as with
  GUILE_SYSTEM_PATH, we have GUILE_SYSTEM_LTDL_PATH.

* meta/uninstalled-env.in: Set GUILE_SYSTEM_LTDL_PATH to "" when
  building.