_tex_source unexpectedly inserted into output filenames

[jhaiduce]

Since fa0376cdac8bccdfac9ef787008ac647d96f6a60, UseLatexMk has been inserting _tex_source into the names of output files. As far as I can tell this happens with every generated file, regardless of whether the filename was specified with the TARGET argument to add_latex_document or derived from the SOURCE argument.

From the commit message this appears to be a workaround needed to compile LaTeX source that uses the minted package, but in most cases users are expecting the output filenames to be the same as the input file, only with .tex replaced with the output file extension (.pdf, .aux, .ps, .dvi, etc.).