PR libstdc++/84654 Disable __float128 specializations for -mno-float128

PR libstdc++/84654 Disable __float128 specializations for -mno-float128

2018-05-01  Tulio Magno Quites Machado Filho  <tuliom@linux.vnet.ibm.com>

	PR libstdc++/84654
	* acinclude.m4: Set ENABLE_FLOAT128 instead of _GLIBCXX_USE_FLOAT128.
	* config.h.in: Remove references to _GLIBCXX_USE_FLOAT128.
	* configure: Regenerate.
	* include/Makefile.am: Replace the value of _GLIBCXX_USE_FLOAT128
	based on ENABLE_FLOAT128.
	* include/Makefile.in: Regenerate.
	* include/bits/c++config: Define _GLIBCXX_USE_FLOAT128.
	[!defined(__FLOAT128__) && !defined(__SIZEOF_FLOAT128__)]: Undefine
	_GLIBCXX_USE_FLOAT128.

From-SVN: r259813