No longer relies on Sys$ReturnCode for swis.h export

Detail:
  amu cleverly sets Sys$ReturnCode to zero just prior to executing
    any command from the Makefile, hence checking for Sys$ReturnCode
    does not work.  Script changed to look for AMU$ReturnCode which
    is (in amu 5.06 and later) the return code of the previous command.
    However, since the variable will not be set by earlier versions of
    amu, the behaviour of always copying the file will return until
    amu 5.06 appears in the build.
Admin:
  Tested manually.

Version 4.99. Tagged as 'RISC_OSLib-4_99'

Makefile

@@ -520,7 +520,7 @@ ${EXP_HDR}.SharedCLib: hdr.SharedCLib
 # exported header.  This prevents needless recompilations
 CLIB:h.swis: derived.swis
 	-${DIFF} $@ derived.swis
-	If "<Sys$ReturnCode>" <> 0 Then ${CP} derived.swis $@ ${CPFLAGS}
+	If "<AMU$ReturnCode>" <> "0" Then ${CP} derived.swis $@ ${CPFLAGS}
 
 # Headers:
 CLIB:h.assert: clib.h.assert  ;   ${CP} clib.h.assert $@ ${CPFLAGS}

VersionASM

@@ -6,9 +6,9 @@
 			GBLS	Module_MinorVersion
 			GBLS	Module_Date
 			GBLS	Module_FullVersion
-Module_MajorVersion	SETS    "4.98"
-Module_Version          SETA    498
+Module_MajorVersion	SETS    "4.99"
+Module_Version          SETA    499
 Module_MinorVersion	SETS	""
 Module_Date		SETS    "25 Nov 1999"
-Module_FullVersion      SETS    "4.98"
+Module_FullVersion      SETS    "4.99"
                         END

VersionNum

-/* (4.98)
+/* (4.99)
  *
  * This file is automatically maintained by srccommit, do not edit manually.
  *
  */
-#define Module_MajorVersion_CMHG     	4.98
+#define Module_MajorVersion_CMHG     	4.99
 #define Module_MinorVersion_CMHG	
 #define Module_Date_CMHG      		25 Nov 1999
 
-#define Module_MajorVersion     	"4.98"
-#define Module_Version                  498
+#define Module_MajorVersion     	"4.99"
+#define Module_Version                  499
 #define Module_MinorVersion		""
 #define Module_Date      		"25 Nov 1999"
 
-#define Module_FullVersion              "4.98"
+#define Module_FullVersion              "4.99"