libtool: Add configuration options for CXX linking

libtool: Add configuration options for CXX linking

Add configuration options to choose whether to use '-nostdlib' when
linking C++ shared libraries and modules.

* m4/libtool.m4: Replace '-nostdlib' with variable '$stdlibflag' and
  set '$output_verbose_link_cmd' to be empty when enabling standard
  library linking for C++ modules.
* m4/ltoptions.m4: Add new configuration options, --enable-cxx-stdlib
  and --disable-cxx-stdlib, and add LT_INIT options, cxx-stdlib and
  no-cxx-stdlib.
* doc/libtool.texi: Update for new configuration options.
* NEWS: Update.