Remove autoconf-2.5x specific m4_default macro.

It is pointless here, because we don't need any "void" statement.
There already is a statement and all it will do in the worst case
is generate an empty line

acinclude.m4

@@ -1341,10 +1341,10 @@ AC_DEFUN(PHP_CHECK_LIBRARY, [
   LDFLAGS="$5 $LDFLAGS"
   AC_CHECK_LIB([$1],[$2],[
     LDFLAGS=$save_old_LDFLAGS
-    m4_default([$3], :)
+    [$3]
   ],[
     LDFLAGS=$save_old_LDFLAGS
-    m4_default([$4], :)
+    [$4]
   ])dnl
 ])