Run makeindex when *.idx files were created during PDF build.

This allows to create indexes easily just by using the latex commands.
The builder takes care that the index is built and the PDF rebuilt properly.

--

For creating an index in a LaTeX PDF the `\index`  and `\printindex` commands are used.
For building the index so that it can include the page numbers of occurrences, one needs to build the PDF (`pdflatex`), then build the index (`makeindex`), then again run `pdflatex` for including the built index in the PDF.

/ @maethu