Initial implementation of dynamic response files.

This implements the ability for response file "@()" substitution to
dynamically adjust to either expand the content or create the response
file depending on the possible command line length. This should
reduce the create of such response temp files improving build
performance.

src/engine/execunix.cpp

@@ -154,7 +154,7 @@ int exec_check
 
     return is_raw_cmd
         ? EXEC_CHECK_OK
-        : check_cmd_for_too_long_lines( command->value, MAXLINE, error_length,
+        : check_cmd_for_too_long_lines( command->value, shell_maxline(), error_length,
             error_max_length );
 }
 
@@ -603,4 +603,9 @@ static int get_free_cmdtab_slot()
     exit( EXITBAD );
 }
 
+int32_t shell_maxline()
+{
+    return MAXLINE;
+}
+
 # endif /* USE_EXECUNIX */