Backport darwin support reorganization from HEAD.
* libtool.m4 [darwin]: Check for nmedit and dsymutil with
AC_CHECK_TOOL, use -single_module by default (can be turned off
with the -multi-module linker flag or by setting the
LT_MULTI_MODULE environment variable. If it is available use
-exported_symbols_list instead of nmedit to remove symbols.
* ltmain.in [darwin]: Use -single_module by default.