Make it impossible to create an invalid BuildSettingCondition.

Instead of a single method that takes two optional values, use multiple methods so that it is impossible for both to be `nil`.

Sources/PackageDescription/BuildSettings.swift

@@ -60,18 +60,27 @@ public struct BuildSettingCondition: Encodable {
 
     /// Creates a build setting condition.
     ///
-    /// At least one parameter is mandatory.
+    /// - Parameters:
+    ///   - platforms: The applicable platforms for this build setting condition.
+    ///   - configuration: The applicable build configuration for this build setting condition.
+    public static func when(platforms: [Platform], configuration: BuildConfiguration) -> BuildSettingCondition {
+        BuildSettingCondition(platforms: platforms, config: configuration)
+    }
+
+    /// Creates a build setting condition.
     ///
     /// - Parameters:
     ///   - platforms: The applicable platforms for this build setting condition.
+    public static func when(platforms: [Platform]) -> BuildSettingCondition {
+        BuildSettingCondition(platforms: platforms, config: .none)
+    }
+
+    /// Creates a build setting condition.
+    ///
+    /// - Parameters:
     ///   - configuration: The applicable build configuration for this build setting condition.
-    public static func when(
-        platforms: [Platform]? = nil,
-        configuration: BuildConfiguration? = nil
-    ) -> BuildSettingCondition {
-        // FIXME: This should be an error, not a precondition.
-        precondition(!(platforms == nil && configuration == nil))
-        return BuildSettingCondition(platforms: platforms, config: configuration)
+    public static func when(configuration: BuildConfiguration) -> BuildSettingCondition {
+        BuildSettingCondition(platforms: .none, config: configuration)
     }
 }