tests/test_xfakekey.c:#include <X11/keysym.h> keysym.h is part of xproto xproto has a pkg-config file called xproto.pc therefore there should be a line like: XDT_CHECK_PACKAGE([XPROTO], [xproto], [7.0]) so it fails at configure time, rather that compile time, if it's missing
Fixed in git master (0ab470c..4f7af80).