There is a problem generating Xcomposite.man and the ebuild dies because of that.
Steps to Reproduce:
1. emerge =x11-libs/libXcomposite-9999
ERROR: x11-libs/libXcomposite-9999 failed:
Created attachment 214233 [details]
Result without app-text/xmlto installed
Created attachment 214235 [details]
Result with app-text/xmlto installed
Created attachment 214236 [details]
Strange. For me it works fine if I manually install xmlto.
This issue is currently being fixed upstream, so there's little for us to do (other than maybe adapt our ebuilds when the final fixes are committed).
Upstream must have fixed something, because this compiles fine for me now.