gdb: Import AX_CXX_COMPILE_STDCXX from the GNU Autoconf Archive

gdb: Import AX_CXX_COMPILE_STDCXX from the GNU Autoconf Archive

This macro throws C++11 code at the compiler in order to check whether
it supports C++11.  final/override, rvalue references, static_assert,
decltype, auto, constexpr, etc., and adds -std=gnu++11 to CXX if
necessary.

Nothing uses the macro yet.  Simply adding it as separate preliminary
step because we'll need local changes.

gdb/ChangeLog
2016-10-28  Pedro Alves  <palves@redhat.com>

	* ax_cxx_compile_stdcxx.m4: New file.