Missing cmake check for Xrender

[Allen Winter]

On Tuesday 27 January 2009 4:46:33 am Josef Spillner wrote:
> Hello,
> 
> my kdelibs cmake checks completed successfully but make then later choked when 
> X11/extensions/Xrender.h was not found in kfilepreviewgenerator.
> A cmake check should be added.
> 
kfilepreviewgenerator unconditionally (for X11 anyway) includes X11/extensions/Xrender.h

So we either
1) hard-require Xrender.
2) make Xrender optional, but then we need someone to look a the code and
    add conditional compiles for X11_Xrender_FOUND.

I volunteer to make the change to the buildsystem, but first I need to know
if we will require Xrender for KDE on X11.  I believe Xrender is optional
in several other KDE modules already.