Work around an LLVM 8.0.0 optimizer bug that causes htmldoc to spin.
https://github.com/michaelrsweet/htmldoc/issues/349
https://bugs.llvm.org/show_bug.cgi?id=41998

Index: htmldoc/ps-pdf.cxx
--- htmldoc/ps-pdf.cxx.orig
+++ htmldoc/ps-pdf.cxx
@@ -5440,7 +5440,7 @@ parse_pre(tree_t *t,		/* I - Tree to parse */
 		line[10240],
 		*lineptr,
 		*dataptr;
-  int		col;
+  volatile int	col;
   float		width,
 		height,
 		rgb[3];