textproc/htmldoc: Add workaround for hangs when compiled with clang 8

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p499421_s346424/logs/errors/htmldoc-1.9.3.log

textproc/htmldoc/files/patch-htmldoc_ps-pdf.cxx

@@ -0,0 +1,15 @@
+Workaround for potential optimizer bug in clang 8 that causes hangs at runtime
+
+https://github.com/michaelrsweet/htmldoc/issues/349
+
+--- htmldoc/ps-pdf.cxx.orig	2018-04-10 11:56:03 UTC
++++ htmldoc/ps-pdf.cxx
+@@ -5361,7 +5361,7 @@ parse_pre(tree_t *t,		/* I - Tree to parse */
+ 		line[10240],
+ 		*lineptr,
+ 		*dataptr;
+-  int		col;
++  volatile int	col;
+   float		width,
+ 		height,
+ 		rgb[3];