bootstrap: Update requirement to C++11.

bootstrap: Update requirement to C++11.

There was general agreement last November that we would move to allowing
C++11 features to be used in GCC 11; this patch implements that direction.

ChangeLog
2020-05-18  Jason Merrill  <jason@redhat.com>

	* configure.ac: Update bootstrap dialect to -std=c++11.

config/ChangeLog
2020-05-18  Jason Merrill  <jason@redhat.com>

	* ax_cxx_compile_stdcxx.m4: Import from autoconf archive with
	an adjustment to try the default mode.

gcc/ChangeLog
2020-05-18  Jason Merrill  <jason@redhat.com>

	* aclocal.m4: Add ax_cxx_compile_stdcxx.m4.
	* configure.ac: Use AX_CXX_COMPILE_STDCXX(11).