If Xwayland is not in /usr/bin, distcheck will fail without patch
023b265b, which propagates @XSERVER_PATH@ to distcheck but blocks other
use of the DISTCHECK_CONFIGURE_FLAGS env var.  Instead, revert the fix
and merely document the need for setting DISTCHECK_CONFIGURE_FLAGS
manually.

Revert "build:  Pass along any user-specified xserver path to distcheck"

This reverts commit 023b265b.