Introduce `UNUSED(x)` macro, mark some variables as unused

src/common.h

@@ -142,4 +142,6 @@ typedef struct registers {
 assert_struct_size(registers, 7 * 4);
 #pragma pack(pop)
 
+#define UNUSED(x) ((void)(x))
+
 #endif

src/openrct2.c

@@ -509,6 +509,9 @@ bool openrct2_setup_rct2_segment()
 	// necessary. Windows does not need to do this as OpenRCT2 runs as a DLL loaded from the Windows PE.
 	int len = 0x01429000 - 0x8a4000; // 0xB85000, 12079104 bytes or around 11.5MB
 	int err = 0;
+	// in some configurations err and len may be unused
+	UNUSED(err);
+	UNUSED(len);
 #if defined(USE_MMAP) && (defined(__unix__) || defined(__MACOSX__)) && !defined(NO_RCT2)
 	#define RDATA_OFFSET 0x004A4000
 	#define DATASEG_OFFSET 0x005E2000