MathJax problem with \gdef

[Christopher Dimech]

Have noticed a problem with including \gdef names in @math,
possibly introduced after the MathJax implementation. Defining
\Desig and calling it from @math{} fails.  It also fails when
doing output to pdf.

@tex
  \gdef\Desig{\buildrel \rm def \over \equiv}
@end tex

@math{\Desig}