memfd-wrappers: only define memfd_create() if not already defined

glibc 2.27 is to be released soon, and it will provide memfd_create().
If glibc provides the function, we must not define it ourselves,
otherwise building fails due to conflict between the two implementations
of the same function.

BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=104733

src/pulsecore/memfd-wrappers.h

@@ -20,13 +20,14 @@
   License along with PulseAudio; if not, see <http://www.gnu.org/licenses/>.
 ***/
 
-#ifdef HAVE_MEMFD
+#if defined(HAVE_MEMFD) && !defined(HAVE_MEMFD_CREATE)
 
 #include <sys/syscall.h>
 #include <fcntl.h>
 
 /*
- * No glibc wrappers exist for memfd_create(2), so provide our own.
+ * Before glibc version 2.27 there was no wrapper for memfd_create(2),
+ * so we have to provide our own.
  *
  * Also define memfd fcntl sealing macros. While they are already
  * defined in the kernel header file <linux/fcntl.h>, that file as
@@ -63,6 +64,6 @@ static inline int memfd_create(const char *name, unsigned int flags) {
 #define F_SEAL_WRITE    0x0008  /* prevent writes */
 #endif
 
-#endif /* HAVE_MEMFD */
+#endif /* HAVE_MEMFD && !HAVE_MEMFD_CREATE */
 
 #endif