Also handle __STDC_WANT_IEC_60559_BFP_EXT__ in <tgmath.h>.

My __STDC_WANT_IEC_60559_BFP_EXT__ patch omitted to update the
conditions on the nextup and nextdown type-generic macros in
<tgmath.h>.  This patch updates those conditions accordingly.  (As
glibc doesn't currently have an exp10 type-generic macro, no such
changes are needed relating to __STDC_WANT_IEC_60559_FUNCS_EXT__;
adding such a type-generic macro would be a new feature.)

Tested for x86_64 and x86 (testsuite, and that installed stripped
shared libraries are unchanged by the patch).  Committed.

	* math/tgmath.h (nextdown): Define if
	[__GLIBC_USE (IEC_60559_BFP_EXT)], not if [__USE_GNU].
	(nextup): Likewise.

ChangeLog

@@ -1,5 +1,9 @@
 2016-08-03  Joseph Myers  <joseph@codesourcery.com>
 
+	* math/tgmath.h (nextdown): Define if
+	[__GLIBC_USE (IEC_60559_BFP_EXT)], not if [__USE_GNU].
+	(nextup): Likewise.
+
 	* bits/libc-header-start.h (__GLIBC_USE_IEC_60559_FUNCS_EXT): New
 	macro.
 	* include/features.h (__STDC_WANT_IEC_60559_FUNCS_EXT__):

math/tgmath.h

@@ -392,7 +392,7 @@
    prevailing rounding mode.  */
 #define rint(Val) __TGMATH_UNARY_REAL_ONLY (Val, rint)
 
-#ifdef __USE_GNU
+#if __GLIBC_USE (IEC_60559_BFP_EXT)
 /* Return X - epsilon.  */
 # define nextdown(Val) __TGMATH_UNARY_REAL_ONLY (Val, nextdown)
 /* Return X + epsilon.  */