[latexmkrc] only use lhs2TeX if it's an lhs file

.latexmkrc

@@ -4,8 +4,11 @@ sub mylatex {
   my $base = shift @_;
   my $tex = "$base.tex";
 
+  # Run the preprocessor
+  if (-e "$base.lhs" && `which lhs2TeX` ne "") {
     # Run the preprocessor
     system('lhs2TeX', '--poly', '-o', $tex, "$base.lhs") == 0 or return $?;
+  }
   # Run pdflatex
   my $return = system('pdflatex', @_, $tex);
   system "echo INPUT $base.lhs >> $aux_dir1$base.fls";