Support __STDC_WANT_IEC_60559_FUNCS_EXT__ feature test macro.

This patch implements support for the
__STDC_WANT_IEC_60559_FUNCS_EXT__ feature test macro, following the
__GLIBC_USE approach used for other ISO C feature test macros.
Currently this only affects the exp10 functions (which glibc has had
for a long time).

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

	* bits/libc-header-start.h (__GLIBC_USE_IEC_60559_FUNCS_EXT): New
	macro.
	* include/features.h (__STDC_WANT_IEC_60559_FUNCS_EXT__):
	Document.
	* manual/creature.texi (__STDC_WANT_IEC_60559_FUNCS_EXT__):
	Document macro.
	* manual/math.texi (exp10): Document as ISO from TS 18661-4:2015.
	(exp10f): Likewise.
	(exp10l): Likewise.
	* math/bits/mathcalls.h (exp10): Declare if
	[__GLIBC_USE (IEC_60559_FUNCS_EXT)], not [__USE_GNU].

1 files changed, 5 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 25c771d..fe9ff1c 100644
--- a/NEWS
+++ b/NEWS
@@ -17,6 +17,11 @@ Version 2.25
   from that TS.  Note that most features from that TS are not supported by
   the GNU C Library.
 
+* The feature test macro __STDC_WANT_IEC_60559_FUNCS_EXT__, from ISO/IEC TS
+  18661-4:2015, is supported to enable declarations of functions and macros
+  from that TS.  Note that most features from that TS are not supported by
+  the GNU C Library.
+
 * The inclusion of <sys/sysmacros.h> by <sys/types.h> is deprecated.  This
   means that in a future release, the macros “major”, “minor”, and “makedev”
   will only be available from <sys/sysmacros.h>.