[Bug libc/21326] C99 functions are not declared for C++11 and later if _GNU_SOURCE is not predefined by g++

["jwakely.gcc at gmail dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=21326

--- Comment #4 from Jonathan Wakely <jwakely.gcc at gmail dot com> ---
--- a/include/features.h
+++ b/include/features.h
@@ -236,13 +236,18 @@
 # define __USE_ISOC95  1
 #endif

+#ifdef __cplusplus
+/* This is to enable compatibility for ISO C++17.  */
+#if (__cplusplus >= 201703L)
+# define __USE_ISOCXX17        1
+# define __USE_ISOC11  1
+#endif
 /* This is to enable compatibility for ISO C++11.
-
-   So far g++ does not provide a macro.  Check the temporary macro for
-   now, too.  */
-#if ((defined __cplusplus && __cplusplus >= 201103L)                         \
-     || defined __GXX_EXPERIMENTAL_CXX0X__)
+   Check the temporary macro for now, too.  */
+#if (__cplusplus >= 201103L || defined __GXX_EXPERIMENTAL_CXX0X__)
 # define __USE_ISOCXX11        1
+# define __USE_ISOC99  1
+#endif
 #endif

 /* If none of the ANSI/POSIX macros are defined, or if _DEFAULT_SOURCE

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