Define SWT_DOCC in the package based on environment variables

Author: grynspan

.github/workflows/pull_request.yml

@@ -25,6 +25,5 @@ jobs:
     with:
       license_header_check_project_name: "Swift"
       docs_check_container_image: "swift:6.2-noble"
-      docs_check_additional_arguments: "-DSWT_DOCC"
       format_check_enabled: false
       api_breakage_check_enabled: false

Package.swift

@@ -36,6 +36,12 @@ let buildingForEmbedded: Bool = {
   return Bool(envvar) ?? ((Int(envvar) ?? 0) != 0)
 }()
 
+/// Whether or not we appear to be building the package for the documentation
+/// soundness check we run in GitHub Actions.
+let buildingForDocumentationSoundnessCheck: Bool = {
+  Context.environment["ADDITIONAL_DOCC_ARGUMENTS"] != nil
+}()
+
 let package = Package(
   name: "swift-testing",
 
@@ -389,6 +395,12 @@ extension Array where Element == PackageDescription.SwiftSetting {
       .define("SWT_NO_LIBDISPATCH", .whenEmbedded()),
     ]
 
+    if buildingForDocumentationSoundnessCheck {
+      result += [
+        .define("SWT_DOCC", .whenEmbedded()),
+      ]
+    }
+
     return result
   }