Add environment variable to force building as dynamic library

Package.swift

@@ -1,9 +1,12 @@
 // swift-tools-version:5.5
 import PackageDescription
+import class Foundation.ProcessInfo
 
-let libraryType: PackageDescription.Product.Library.LibraryType = .dynamic
+// force building as dynamic library
+let dynamicLibrary = ProcessInfo.processInfo.environment["SWIFT_BUILD_DYNAMIC_LIBRARY"] != nil
+let libraryType: PackageDescription.Product.Library.LibraryType? = dynamicLibrary ? .dynamic : nil
 
-let package = Package(
+var package = Package(
     name: "BluetoothLinux",
     platforms: [
         .macOS(.v10_15),
@@ -59,3 +62,13 @@ let package = Package(
         )
     ]
 )
+
+// SwiftPM command plugins are only supported by Swift version 5.6 and later.
+#if swift(>=5.6)
+let buildDocs = ProcessInfo.processInfo.environment["BUILDING_FOR_DOCUMENTATION_GENERATION"] != nil
+if buildDocs {
+    package.dependencies += [
+        .package(url: "https://github.com/apple/swift-docc-plugin", from: "1.0.0"),
+    ]
+}
+#endif