build: pass YAML_DEFINE_STATIC to the CInterop (#398)

While we would now build CYaml statically, we would fail to indicate to
clang the library was meant to be used statically and not dynamically.
This cleans up some linker warnings when building SPM.

Package.swift

@@ -16,7 +16,8 @@ let package = Package(
         .target(
             name: "Yams",
             dependencies: ["CYaml"],
-            exclude: ["CMakeLists.txt"]
+            exclude: ["CMakeLists.txt"],
+            cSettings: [.define("YAML_DECLARE_STATIC")]
         ),
         .testTarget(
             name: "YamsTests",