Fix CompileCommandOutput test build on Windows

Add dllexport markup for the shared library.

Tests/CompileCommandOutput/relative.h

@@ -1 +1,11 @@
-void relative();
+#if defined(_WIN32)
+# ifdef test2_EXPORTS
+#  define TEST2_EXPORT __declspec(dllexport)
+# else
+#  define TEST2_EXPORT __declspec(dllimport)
+# endif
+#else
+# define TEST2_EXPORT
+#endif
+
+TEST2_EXPORT void relative();