Allow compiling with markdown3

Currently compilation fails with:

    /build/src/classes/renderer/markdown.c:191:20: error: variable or field 'flags' declared void

According to comment [1], markdown3 introduces backward
incompatibility with respect to document flags.

This commit adds conditionally compiled code that allows building
pdfpc with both markdown versions 2 and 3.

The used cmake operator VERSION_GREATER_EQUAL requires cmake 3.7 so we
bump minimal cmake version requirement.

Fixes pdfpc#682

[1]: pdfpc#682 (comment)

Author: wentasah

CMakeLists.txt

@@ -1,5 +1,5 @@
 project("pdfpc" C)
-cmake_minimum_required(VERSION 3.0)
+cmake_minimum_required(VERSION 3.7)
 
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake/vala)
 

src/CMakeLists.txt

@@ -32,6 +32,10 @@ if (MDVIEW OR REST)
     pkg_check_modules(MARKDOWN REQUIRED libmarkdown)
 endif ()
 
+if ("${MARKDOWN_VERSION}" VERSION_GREATER_EQUAL 3)
+    set(EXTRA_VALA_OPTIONS ${EXTRA_VALA_OPTIONS} -D MARKDOWN3)
+endif ()
+
 if (MDVIEW)
     pkg_check_modules(WEBKIT REQUIRED webkit2gtk-4.0)
     set(MDVIEW_PACKAGES webkit2gtk-4.0)

src/classes/renderer/markdown.vala

@@ -23,7 +23,12 @@
 namespace pdfpc.Renderer {
     public class MD {
         public static string render(string? text = "", bool plain_text = false) {
+#if MARKDOWN3
+            var flags = new Markdown.DocumentFlags();
+            flags.set(Markdown.DocumentFlag.NO_EXT);
+#else
             Markdown.DocumentFlags flags = Markdown.DocumentFlags.NO_EXT;
+#endif
 
             string html;
             if (text != "" && plain_text) {

src/libmarkdown.vapi

@@ -95,9 +95,23 @@ namespace Markdown
 		public void ref_prefix (string prefix);
 	}
 
+#if MARKDOWN3
+	[Compact]
+	[CCode (cname = "mkd_flag_t", free_function = "mkd_free_flags")]
+	public class DocumentFlags {
+		[CCode (cname = "mkd_flags")]
+        public DocumentFlags();
+		[CCode (cname = "mkd_set_flag_num")]
+		public void set (DocumentFlag flag);
+    }
+
+	[CCode (cprefix = "MKD_")]
+	public enum DocumentFlag
+#else
 	[Flags]
 	[CCode (cname = "mkd_flag_t", cprefix = "MKD_")]
 	public enum DocumentFlags
+#endif
 	{
 		NOLINKS,
 		NOIMAGE,