Fix to make it build on Swift 4.2

Package@swift-4.swift

@@ -0,0 +1,14 @@
+// swift-tools-version:4.0
+
+import PackageDescription
+
+let package = Package(
+    name: "cmark",
+    products: [
+        .library(name: "cmark", targets: ["cmark"]),
+    ],
+    targets: [
+        .target(name: "cmark")
+    ]
+)
+

Sources/cmark/ext_scanners.re

@@ -0,0 +1,74 @@
+#include <stdlib.h>
+#include "ext_scanners.h"
+
+bufsize_t _ext_scan_at(bufsize_t (*scanner)(const unsigned char *), unsigned char *ptr, int len, bufsize_t offset)
+{
+	bufsize_t res;
+
+        if (ptr == NULL || offset > len) {
+          return 0;
+        } else {
+	  unsigned char lim = ptr[len];
+
+	  ptr[len] = '\0';
+	  res = scanner(ptr + offset);
+	  ptr[len] = lim;
+        }
+
+	return res;
+}
+
+/*!re2c
+  re2c:define:YYCTYPE  = "unsigned char";
+  re2c:define:YYCURSOR = p;
+  re2c:define:YYMARKER = marker;
+  re2c:define:YYCTXMARKER = marker;
+  re2c:yyfill:enable = 0;
+
+  spacechar = [ \t\v\f];
+  newline = [\r]?[\n];
+  escaped_char = [\\][|!"#$%&'()*+,./:;<=>?@[\\\]^_`{}~-];
+
+  table_marker = (spacechar*[:]?[-]+[:]?spacechar*);
+  table_cell = (escaped_char|[^|\r\n])*;
+*/
+
+bufsize_t _scan_table_start(const unsigned char *p)
+{
+  const unsigned char *marker = NULL;
+  const unsigned char *start = p;
+/*!re2c
+  [|]? table_marker ([|] table_marker)* [|]? spacechar* newline { return (bufsize_t)(p - start); }
+  .? { return 0; }
+*/
+}
+
+bufsize_t _scan_table_cell(const unsigned char *p)
+{
+  const unsigned char *marker = NULL;
+  const unsigned char *start = p;
+/*!re2c
+  table_cell { return (bufsize_t)(p - start); }
+  .? { return 0; }
+*/
+}
+
+bufsize_t _scan_table_cell_end(const unsigned char *p)
+{
+  const unsigned char *marker = NULL;
+  const unsigned char *start = p;
+/*!re2c
+  [|] spacechar* newline? { return (bufsize_t)(p - start); }
+  .? { return 0; }
+*/
+}
+
+bufsize_t _scan_table_row_end(const unsigned char *p)
+{
+  const unsigned char *marker = NULL;
+  const unsigned char *start = p;
+/*!re2c
+  spacechar* newline { return (bufsize_t)(p - start); }
+  .? { return 0; }
+*/
+}

Sources/cmark/include/cmark.h

@@ -2,8 +2,8 @@
 #define CMARK_CMARK_H
 
 #include <stdio.h>
-#include "../cmark_export.h"
-#include "../cmark_version.h"
+#include "cmark_export.h"
+#include "cmark_version.h"
 
 #ifdef __cplusplus
 extern "C" {