fbc: error message for undefined built-in symbol

- if '-z nobuiltins' or some built-in procedure was
  undefined (#undef by user), then show a special
  message to indicate that a built-in symbol is
  undefined

src/compiler/error.bas

@@ -430,6 +430,7 @@ declare function hMakeParamDesc _
 		/'FB_ERRMSG_GFXLIBNOTSUPPORTEDFORTARGET        '/ @"Graphics routines were used, but the gfxlib has not been ported to this target", _
 		/'FB_ERRMSG_SSEREQUIRESX86                     '/ @"-fpu sse option can only be used on x86 and x86_64 architectures", _
 		/'FB_ERRMSG_NEONREQUIRESARM                    '/ @"-fpu neon option can only be used on arm architectures", _
+		/'FB_ERRMSG_UNDEFINEDBUILTINSYMBOL             '/ @"Undefined built-in symbol", _
 		/'FB_ERRMSGS                                   '/ @"FB_ERRMSGS"  _
 	}
 

src/compiler/error.bi

@@ -335,6 +335,7 @@ enum FB_ERRMSG
 	FB_ERRMSG_GFXLIBNOTSUPPORTEDFORTARGET
 	FB_ERRMSG_SSEREQUIRESX86
 	FB_ERRMSG_NEONREQUIRESARM
+	FB_ERRMSG_UNDEFINEDBUILTINSYMBOL
 
 	FB_ERRMSGS
 end enum

src/compiler/rtl.bas

@@ -309,7 +309,7 @@ function rtlProcLookup _
 				pname = strptr( tmp_name )
 				chain_ = symbLookupAt( @symbGetGlobalNamespc( ), pname, FALSE, FALSE )
 				if( chain_ = NULL ) then
-					errReportEx( FB_ERRMSG_UNDEFINEDSYMBOL, *pname )
+					errReportEx( FB_ERRMSG_UNDEFINEDBUILTINSYMBOL, *pname )
 					rtlLookupTB( pidx ) = NULL
 				else
 					rtlLookupTB( pidx ) = chain_->sym