Enforce C++11 as a minimum for building gold [PR30867]

Enforce C++11 as a minimum for building gold [PR30867]

The attempt in 5e9091dab885 to correct gold for modern LLVM has broken
gold for older compilers.  This commit introduced C++11 types without
changing the build system to require a C++ compiler.  More importantly
it depends on the compiler having at least C++11 as the default
language.  Older compilers which support C++11 but not as the default
language needlessly break.  Fix that.

	PR gold/30867
	* configure.ac (AX_CXX_COMPILE_STDCXX): Require C++11.
	* Makefile.in: Regenerate.
	* aclocal.m4: Regenerate.
	* config.in: Regenerate.
	* configure: Regenerate.
	* testsuite/Makefile.in: Regenerate.