Link $aux_dir back from latexmkrc to auctex

[cxandru]

Auctex is unaware of the $aux_dir I specified in my latexmkrc, which means the "go to error" command doesn't work because it can't find the log file. Could this option be linked back to auctex? Kind of the opposite of #45 in the sense that we want to communicate external latexmk configuration options back to auctex.