bootstrap: Fix --disable-bootstrap with older g++.

bootstrap: Fix --disable-bootstrap with older g++.

Previously I had AX_CXX_COMPILE_STDCXX in the gcc directory configure, which
added -std=c++11 to CXX if needed, but then CXX is overridden from the
toplevel directory, so it didn't have the desired effect.  Fixed by moving
the check to the toplevel.  Currently it is only used when building GCC
without bootstrapping; other packages that share the toplevel directory
can adjust the condition if they also want to require C++11 support.

/ChangeLog:

	* configure.ac: Check AX_CXX_COMPILE_STDCXX if not bootstrapping.
	* configure: Regenerate.

gcc/ChangeLog:

	* aclocal.m4: Remove ax_cxx_compile_stdcxx.m4.
	* configure.ac: Remove AX_CXX_COMPILE_STDCXX.
	* configure: Regenerate.