Sort input file list

so that variables_f.js builds in a reproducible way
in spite of indeterministic filesystem readdir order.

See https://reproducible-builds.org/ for why this is good.

Fixes trilinos#5559

doc/build_docs.pl

@@ -37,7 +37,7 @@
 print "\n";
 if(!$skipBuild) {
   print "\nGenerating package documentation ...\n";
-  find (\&buildDocs, $trilinosDir);
+  find ({wanted=>\&buildDocs, preprocess => sub {sort {$a cmp $b} @_}}, $trilinosDir);
   if($build_dir) {
     find (\&copyXML, $trilinosDir);
   }