#323 Create aux_directory if it doesn't exist (pdflatex)

[vikusz]

I like having aux files in a separate folder ("tex_aux" subfolder of the current directory). This can be achieved trivially setting "-aux-directory=.\tex_aux" in pdflatex execution option. However if the tex_aux subdirectory doesn't exist TeXstudio returns an error and the compilation stops. One has to create a tex_aux subfolder for every project manually.

It would be nice if the set aux_directory was created automatically in case it doesn't exist.