build: hoist CMARK_THREADING to a build-only config

This macro does not impact the public API surface and the user should
not have to care. This removes the macro from the configuration headers.

CMakeLists.txt

@@ -21,6 +21,7 @@ include(FindAsan)
 if(CMARK_THREADING)
   set(THREADS_PREFER_PTHREAD_FLAG YES)
   include(FindThreads)
+  add_compile_definitions(CMARK_THREADING)
 endif()
 include(GNUInstallDirs)
 

Package.swift

@@ -11,9 +11,12 @@ import PackageDescription
     // link time.
     let cSettings: [CSetting] = [
         .define("CMARK_GFM_STATIC_DEFINE", .when(platforms: [.windows])),
+        .define("CMARK_THREADING"),
     ]
 #else
-    let cSettings: [CSetting] = []
+    let cSettings: [CSetting] = [
+        .define("CMARK_THREADING"),
+    ]
 #endif
 
 let package = Package(

src/config.h.in

@@ -5,8 +5,6 @@
 extern "C" {
 #endif
 
-#cmakedefine CMARK_THREADING
-
 #ifdef __cplusplus
 }
 #endif

src/include/cmark-gfm_config.h

@@ -12,8 +12,6 @@
 extern "C" {
 #endif
 
-#define CMARK_THREADING
-
 #ifdef __cplusplus
 }
 #endif