Restore the behaviour in GCC 8 and earlier where _GLIBCXX_USE_FLOAT128
is not defined when configure detects support is missing. This avoids
having three states where the macro is either 1, 0, or undefined.

	PR libstdc++/85672
	* include/Makefile.am [!ENABLE_FLOAT128]: Change c++config.h entry
	to #undef _GLIBCXX_USE_FLOAT128 instead of defining it to zero.
	* include/Makefile.in: Regenerate.
	* include/bits/c++config (_GLIBCXX_USE_FLOAT128): Move definition
	within conditional block.

From-SVN: r260043