re PR libstdc++/64967 (Bootstrap fails due to errors in libstdc++ sources with `--enable-symvers=gnu-versioned-namespace')

	PR libstdc++/64967
	* acinclude.m4: Disable dual ABI when gnu-versioned-namespace in use.
	* configure: Regenerate.
	* src/c++11/compatibility-c++0x.cc (error_category), generic_category,
	system_category): Use macros for versioned namespace.
	* src/c++11/futex.cc: Add missing end macro for versioned namespace.

From-SVN: r221600