Enable cmark to build as a SwiftPM package

Add a `Package.swift` and some extra files that are cmake-generated.

Package.swift

@@ -0,0 +1,21 @@
+// swift-tools-version:4.0
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+  name: "cmark",
+  products: [
+    .library(
+      name: "cmark",
+      targets: ["cmark"]),
+  ],
+  targets: [
+    .target(
+      name: "cmark",
+      path: "src",
+      // Exclude the main file so cmark is built as a library.
+      exclude: ["main.c"]
+    )
+  ]
+)

src/config.h

@@ -0,0 +1,76 @@
+#ifndef CMARK_CONFIG_H
+#define CMARK_CONFIG_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define HAVE_STDBOOL_H
+
+#ifdef HAVE_STDBOOL_H
+  #include <stdbool.h>
+#elif !defined(__cplusplus)
+  typedef char bool;
+#endif
+
+#define HAVE___BUILTIN_EXPECT
+
+#define HAVE___ATTRIBUTE__
+
+#ifdef HAVE___ATTRIBUTE__
+  #define CMARK_ATTRIBUTE(list) __attribute__ (list)
+#else
+  #define CMARK_ATTRIBUTE(list)
+#endif
+
+#ifndef CMARK_INLINE
+  #if defined(_MSC_VER) && !defined(__cplusplus)
+    #define CMARK_INLINE __inline
+  #else
+    #define CMARK_INLINE inline
+  #endif
+#endif
+
+/* snprintf and vsnprintf fallbacks for MSVC before 2015,
+   due to Valentin Milea http://stackoverflow.com/questions/2915672/
+*/
+
+#if defined(_MSC_VER) && _MSC_VER < 1900
+
+#include <stdio.h>
+#include <stdarg.h>
+
+#define snprintf c99_snprintf
+#define vsnprintf c99_vsnprintf
+
+CMARK_INLINE int c99_vsnprintf(char *outBuf, size_t size, const char *format, va_list ap)
+{
+    int count = -1;
+
+    if (size != 0)
+        count = _vsnprintf_s(outBuf, size, _TRUNCATE, format, ap);
+    if (count == -1)
+        count = _vscprintf(format, ap);
+
+    return count;
+}
+
+CMARK_INLINE int c99_snprintf(char *outBuf, size_t size, const char *format, ...)
+{
+    int count;
+    va_list ap;
+
+    va_start(ap, format);
+    count = c99_vsnprintf(outBuf, size, format, ap);
+    va_end(ap);
+
+    return count;
+}
+
+#endif
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif

src/include/cmark.h

@@ -0,0 +1 @@
+../cmark.h

src/include/cmark_export.h

@@ -0,0 +1,42 @@
+
+#ifndef CMARK_EXPORT_H
+#define CMARK_EXPORT_H
+
+#ifdef CMARK_STATIC_DEFINE
+#  define CMARK_EXPORT
+#  define CMARK_NO_EXPORT
+#else
+#  ifndef CMARK_EXPORT
+#    ifdef libcmark_EXPORTS
+        /* We are building this library */
+#      define CMARK_EXPORT __attribute__((visibility("default")))
+#    else
+        /* We are using this library */
+#      define CMARK_EXPORT __attribute__((visibility("default")))
+#    endif
+#  endif
+
+#  ifndef CMARK_NO_EXPORT
+#    define CMARK_NO_EXPORT __attribute__((visibility("hidden")))
+#  endif
+#endif
+
+#ifndef CMARK_DEPRECATED
+#  define CMARK_DEPRECATED __attribute__ ((__deprecated__))
+#endif
+
+#ifndef CMARK_DEPRECATED_EXPORT
+#  define CMARK_DEPRECATED_EXPORT CMARK_EXPORT CMARK_DEPRECATED
+#endif
+
+#ifndef CMARK_DEPRECATED_NO_EXPORT
+#  define CMARK_DEPRECATED_NO_EXPORT CMARK_NO_EXPORT CMARK_DEPRECATED
+#endif
+
+#if 0 /* DEFINE_NO_DEPRECATED */
+#  ifndef CMARK_NO_DEPRECATED
+#    define CMARK_NO_DEPRECATED
+#  endif
+#endif
+
+#endif /* CMARK_EXPORT_H */

src/include/cmark_version.h

@@ -0,0 +1,7 @@
+#ifndef CMARK_VERSION_H
+#define CMARK_VERSION_H
+
+#define CMARK_VERSION ((0 << 16) | (28 << 8) | 3)
+#define CMARK_VERSION_STRING "0.28.3"
+
+#endif