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, 4 deletions

diff --git a/manual/math.texi b/manual/math.texi
index 5c9f7b9..d689820 100644
--- a/manual/math.texi
+++ b/manual/math.texi
@@ -480,13 +480,13 @@ Mathematically, @code{exp2 (x)} is the same as @code{exp (x * log (2))}.
 @end deftypefun
 
 @comment math.h
-@comment GNU
+@comment ISO
 @deftypefun double exp10 (double @var{x})
 @comment math.h
-@comment GNU
+@comment ISO
 @deftypefunx float exp10f (float @var{x})
 @comment math.h
-@comment GNU
+@comment ISO
 @deftypefunx {long double} exp10l (long double @var{x})
 @comment math.h
 @comment GNU
@@ -501,7 +501,8 @@ Mathematically, @code{exp2 (x)} is the same as @code{exp (x * log (2))}.
 These functions compute @code{10} raised to the power @var{x}.
 Mathematically, @code{exp10 (x)} is the same as @code{exp (x * log (10))}.
 
-These functions are GNU extensions.  The name @code{exp10} is
+The @code{exp10} functions are from TS 18661-4:2015; the @code{pow10}
+names are GNU extensions.  The name @code{exp10} is
 preferred, since it is analogous to @code{exp} and @code{exp2}.
 @end deftypefun