rust: avoid clobbering LIBS

Save LIBS around calls to AC_SEARCH_LIBS to avoid clobbering $LIBS.

ChangeLog:

	* configure: Regenerate.
	* configure.ac: Save LIBS around calls to AC_SEARCH_LIBS.

Signed-off-by: Marc Poulhiès <dkm@kataplop.net>
Reviewed-by: Thomas Schwinge <tschwinge@baylibre.com>
Tested-by: Thomas Schwinge <tschwinge@baylibre.com>