* corrected handling of validation of math macros. The macro instances

* corrected handling of validation of math macros. The macro instances
  will require the needed LaTeXFeatures of their definition and their
  paramenters.
* Require the "xargs" package when there is a macro with optional
  parameters which is prepended to a child document which is rendered alone.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@23971 a592a061-630c-0410-9148-cb99ea01b6c8