Use the right latex backend when a converter needs aux files.

Use the right latex backend when a converter needs aux files.

If a converter specifies the needaux flag, latex (or xelatex) is
always run to produce the needed auxiliary files. This is wrong
because there are documents that can only be compiled with a specific
backend and thus the conversion may fail. On the other hand, even if
the document specifies the backend to be used, LyX ignores this info.

This commit rectifies this behavior by letting LyX run the same flavor
of the latex backend that shall be used for previewing the document
also for producing the auxiliary files.