MaterialX's cmake config should find_dependency its own OpenGL and X11 dependencies

[spiffmon]

Hi, please have a look at OpenUSD Issue 3648, which is a proposed fix to USD's cmake to work around the lack in MaterialX, when building USD with MaterialX. It would be nice if we didn't need to intuit the transitive dependencies ourselves, and wonder if other MaterialX users haven't run into this?

Thanks! --spiff