libstdc++: Allow 'configure.host' to pre-set 'EXTRA_CFLAGS', 'EXTRA_CXX_FLAGS'

In particular, 'GLIBCXX_ENABLE_CXX_FLAGS' shouldn't overwrite 'EXTRA_CXX_FLAGS'
(and instead append any additional '--enable-cxx-flags=[...]').

	libstdc++-v3/
	* acinclude.m4 (GLIBCXX_ENABLE_CXX_FLAGS): Append to
	'EXTRA_CXX_FLAGS' any additional flags.
	* configure: Regenerate.
	* configure.host: Document 'EXTRA_CFLAGS', 'EXTRA_CXX_FLAGS'.