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 *));

include/pcconf.h

@@ -35,7 +35,7 @@
 
 /*#define OVERLAY */ /* Manual overlay definition (MSC 6.0ax only) */
 
-#ifndef AMIGA_CROSS
+#ifndef CROSS_TO_AMIGA
 #define SHELL /* via exec of COMMAND.COM */
 #endif
 
@@ -90,7 +90,7 @@
 #define ANSI_DEFAULT
 #endif
 
-#ifndef AMIGA_CROSS
+#ifndef CROSS_TO_AMIGA
 #define RANDOM /* have Berkeley random(3) */
 #endif
 
@@ -105,7 +105,7 @@
 #include <process.h> /* Provides prototypes of exit(), spawn()      */
 #endif
 
-#ifdef AMIGA_CROSS
+#ifdef CROSS_TO_AMIGA
 #include <spawn.h>
 #endif