build: fix build failure on AIX

* m4/jm-macros.m4: AIX has a splice() function for TCP,
so check for vmsplice() instead.
* src/splice.h: Define HAVE_SPLICE if vmsplice available.
Reported by Bruno Haible.

m4/jm-macros.m4

@@ -70,7 +70,7 @@ AC_DEFUN([coreutils_MACROS],
     setgroups
     sethostname
     siginterrupt
-    splice
+    vmsplice
     sync
     sysinfo
     tcgetpgrp

src/splice.h

@@ -17,7 +17,12 @@
 #ifndef SPLICE_H
 # define SPLICE_H 1
 
-# if HAVE_SPLICE
+# if HAVE_VMSPLICE
+
+/* splice() and vmsplice() were introduced at the same time,
+   so assume splice() is available.  Note AIX also has a different
+   splice() which is why we don't explicitly check for that function. */
+#  define HAVE_SPLICE 1
 
 /* Empirically determined pipe size for best throughput.
    Needs to be <= /proc/sys/fs/pipe-max-size  */