Based on https://gitweb.git.savannah.gnu.org/gitweb/?p=gnulib.git;a=commitdiff_plain;h=c44fe03b72687c9e913727724c29bdb49c1f86e3

Backported to 5c5298c4f47bf1912f6317a938df501e680f2bcb for use in packages with older gnulibs

diff --git a/lib/string.in.h b/lib/string.in.h
index e764221168..db574ca576 100644
--- a/lib/string.in.h
+++ b/lib/string.in.h
@@ -214,38 +214,58 @@ _GL_EXTERN_C void free (void *);
 
 /* Declarations for ISO C N3322.  */
 #if defined __GNUC__ && __GNUC__ >= 15 && !defined __clang__
+# ifndef memcpy
 _GL_EXTERN_C void *memcpy (void *__dest, const void *__src, size_t __n)
   _GL_ATTRIBUTE_NONNULL_IF_NONZERO (1, 3)
   _GL_ATTRIBUTE_NONNULL_IF_NONZERO (2, 3);
+# endif
+# ifndef memccpy
 _GL_EXTERN_C void *memccpy (void *__dest, const void *__src, int __c, size_t __n)
   _GL_ATTRIBUTE_NONNULL_IF_NONZERO (1, 4)
   _GL_ATTRIBUTE_NONNULL_IF_NONZERO (2, 4);
+# endif
+# ifndef memmove
 _GL_EXTERN_C void *memmove (void *__dest, const void *__src, size_t __n)
   _GL_ATTRIBUTE_NONNULL_IF_NONZERO (1, 3)
   _GL_ATTRIBUTE_NONNULL_IF_NONZERO (2, 3);
+# endif
+# ifndef strncpy
 _GL_EXTERN_C char *strncpy (char *__dest, const char *__src, size_t __n)
   _GL_ATTRIBUTE_NONNULL_IF_NONZERO (1, 3)
   _GL_ATTRIBUTE_NONNULL_IF_NONZERO (2, 3);
+# endif
+# ifndef strndup
 _GL_EXTERN_C char *strndup (const char *__s, size_t __n)
   _GL_ATTRIBUTE_NONNULL_IF_NONZERO (1, 2);
+# endif
+# ifndef strncat
 _GL_EXTERN_C char *strncat (char *__dest, const char *__src, size_t __n)
   _GL_ARG_NONNULL ((1)) _GL_ATTRIBUTE_NONNULL_IF_NONZERO (2, 3);
+# endif
+# ifndef memcmp
 _GL_EXTERN_C int memcmp (const void *__s1, const void *__s2, size_t __n)
   _GL_ATTRIBUTE_NONNULL_IF_NONZERO (1, 3)
   _GL_ATTRIBUTE_NONNULL_IF_NONZERO (2, 3);
+# endif
+# ifndef strncmp
 _GL_EXTERN_C int strncmp (const char *__s1, const char *__s2, size_t __n)
   _GL_ATTRIBUTE_NONNULL_IF_NONZERO (1, 3)
   _GL_ATTRIBUTE_NONNULL_IF_NONZERO (2, 3);
-# ifndef __cplusplus
+# endif
+# if !defined memchr && !defined __cplusplus
 _GL_EXTERN_C void *memchr (const void *__s, int __c, size_t __n)
   _GL_ATTRIBUTE_NONNULL_IF_NONZERO (1, 3);
 _GL_EXTERN_C void *memrchr (const void *__s, int __c, size_t __n)
   _GL_ATTRIBUTE_NONNULL_IF_NONZERO (1, 3);
 # endif
+# ifndef memset
 _GL_EXTERN_C void *memset (void *__s, int __c, size_t __n)
   _GL_ATTRIBUTE_NONNULL_IF_NONZERO (1, 3);
+# endif
+# ifndef memset_explicit
 _GL_EXTERN_C void *memset_explicit (void *__s, int __c, size_t __n)
   _GL_ATTRIBUTE_NONNULL_IF_NONZERO (1, 3);
+# endif
 #endif