define strings to prevent warnings

configure

@@ -604,6 +604,9 @@ sub secondary_variables
             $precomp_use_flags='';
         }
     }
+    $precomp_file //= '';
+    $precomp_build_flags //= '';
+    $precomp_use_flags //= '';
     
 
     my $nativecheck = $cflags=~/-mtune=native/ ? ' (using native flag for -mcpu and -mtune)' :'';