Support LaTeX *ifthen* package

[tarleb]

The description on CTAN states:

The package’s basic command is \ifthenelse, which can use a wide array of tests. Also provided is a simple loop command \whiledo.

Ifthen is a separate package within the LaTeX distribution; while it will always be present in a LaTeX distribution, a \usepackage command is always needed to load it.

This issue is a replacement for #8255.