Factor out common usage-help code

include/platform.hpp

@@ -1,7 +1,5 @@
 // SPDX-License-Identifier: MIT
 
-// platform-specific hacks
-
 #ifndef RGBDS_PLATFORM_HPP
 #define RGBDS_PLATFORM_HPP
 

include/usage.hpp

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: MIT
+
+#ifndef RGBDS_USAGE_HPP
+#define RGBDS_USAGE_HPP
+
+#include <stdarg.h>
+
+class Usage {
+	char const *usage;
+
+public:
+	Usage(char const *usage_) : usage(usage_) {}
+
+	[[noreturn]]
+	void printAndExit(int code) const;
+
+	[[gnu::format(printf, 2, 3), noreturn]]
+	void printAndExit(char const *fmt, ...) const;
+};
+
+#endif // RGBDS_USAGE_HPP