Sessreg looks for x11.pc during configure, but a cursory examination of the
build doesn't indicate any need for libX11. However, it does use bits from
xproto. Maybe the author meant xproto instead of x11?
Sorry about the phenomenal bug spam, guys. Adding xorg-team@ to the QA contact so bugs don't get lost in future.
From what I see this is right. sessreg only needs xproto, not x11.
Fix pushed to git. Thanks for your report!