add $libir to the ltdl path, not $pkglibdir; add extensionsdir

add $libir to the ltdl path, not $pkglibdir; add extensionsdir

* libguile/Makefile.am (libpath.h): Fix SCM_LIB_DIR to point to the
  libdir, not pkglibdir. Add SCM_EXTENSIONS_DIR, for a dir to put in the
  dynamic-link search path that's not managed by ldconfig or the like;
  and put this dir in the "extensionsdir" key of %guile-build-info.

* libguile/dynl.c (sysdep_dynl_init): Rename the installed-libdir
  environment var to GUILE_SYSTEM_EXTENSIONS_PATH (from
  GUILE_SYSTEM_LTDL_PATH). Also add SCM_EXTENSIONS_DIR to the ltdl
  search path in the default case.
* meta/uninstalled-env.in: Adapt override to match.

* meta/guile-2.0.pc.in: Add extensionsdir, so you can pkg-config
  --variable=extensionsdir guile-2.0.