Fix Makefile warning about circular empty.h dependency.

In commit be586d53b I made empty.h depend on $(allsources), which
unfortunately was defined so as to include empty.h. This was harmless,
because make just ignored the circular dependency, but annoying,
because it constantly mentioned that it was ignoring it.

mkfiles.pl

@@ -1517,7 +1517,8 @@ if (defined $makefiles{'am'}) {
     # auto-generated parts of this makefile, but Recipe might like to
     # have it available as a variable so that mandatory-rebuild things
     # (version.o) can conveniently be made to depend on it.
-    @sources = ("allsources", "=", sort keys %allsourcefiles);
+    @sources = ("allsources", "=",
+                sort grep {$_ ne "empty.h"} keys %allsourcefiles);
     print &splitline(join " ", @sources), "\n\n";
 
     @cliprogs = ("bin_PROGRAMS", "=");