be more consistent with CROSS_TO_target macro names for cross-compiles

-DCROSS_TO_MSDOS	msdos cross-compile (djgpp cross-compiler)
-DCROSS_TO_AMIGA	Amiga cross-compile
-DCROSS_TO_WASM		wasm cross-compile (emscripten)

include/extern.h

@@ -2150,7 +2150,7 @@ E void NDECL(deliver_splev_message);
 /* ### random.c ### */
 
 #if defined(RANDOM) && !defined(__GO32__) /* djgpp has its own random */
-#ifndef AMIGA_CROSS
+#ifndef CROSS_TO_AMIGA
 E void FDECL(srandom, (unsigned));
 E char *FDECL(initstate, (unsigned, char *, int));
 E char *FDECL(setstate, (char *));