libtool: take care not to double-apply LT_SYS_LIBRARY_PATH.

* m4/libtool.m4 (_LT_CONFIG_SAVE_COMMANDS): Copy configure-time
LT_SYS_LIBRARY_PATH settings as default, but allow run-time
override.
(_LT_SYS_DYNAMIC_LINKER): Save the unmunged
sys_lib_dlsearch_path_spec value, and use it for _LT_DECL,
but then munge it with LT_SYS_LIBRARY_PATH for use in ltdl.m4
macros, such as LT_SYS_DLSEARCH_PATH.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>