I've switched the continuous delivery pipeline for GNOME to use Meson when building the X server, and when I tried disabling the dependency on libXdmcp using the `-Dxdmcp=false` option, I got:

```
FAILED: os/libxserver_os@sta/auth.c.o 
ccache cc  -Ios/libxserver_os@sta -Ios -I../os -Ixfixes -I../xfixes -Irender -I../render -Irandr -I../randr -Ipresent -I../present -Iinclude -I../include -Idri3 -I../dri3 -Idbe -I../dbe -Imiext/sync -I../miext/sync -Imiext/shadow -I../miext/shadow -Imiext/damage -I../miext/damage -Imi -I../mi -Iglamor -I../glamor -Ifb -I../fb -Iexa -I../exa -Idamageext -I../damageext -Icomposite -I../composite -IXi -I../Xi -IXext -I../Xext -I/usr/include/X11/dri -I/usr/include/pixman-1 -I/usr/include/freetype2 -I/usr/include/libpng16 -I/usr/include/harfbuzz -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -fdiagnostics-color=always -pipe -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -std=gnu99 -O2 -g -DHAVE_DIX_CONFIG_H -fno-strict-aliasing -fvisibility=hidden -Wall -Wpointer-arith -Wmissing-declarations -Wformat=2 -Wstrict-prototypes -Wmissing-prototypes -Wnested-externs -Wbad-function-cast -Wold-style-definition -Wunused -Wuninitialized -Wshadow -Wmissing-noreturn -Wmissing-format-attribute -Wredundant-decls -Werror=implicit -Werror=nonnull -Werror=init-self -Werror=main -Werror=missing-braces -Werror=sequence-point -Werror=return-type -Werror=trigraphs -Werror=array-bounds -Werror=write-strings -Werror=address -Werror=int-to-pointer-cast -Werror=pointer-to-int-cast -O2 -g -m64 -mtune=generic -fPIC -D_DEFAULT_SOURCE -D_BSD_SOURCE -DHAS_FCHOWN -DHAS_STICKY_DIR_BIT -MMD -MQ 'os/libxserver_os@sta/auth.c.o' -MF 'os/libxserver_os@sta/auth.c.o.d' -o 'os/libxserver_os@sta/auth.c.o' -c ../os/auth.c
In file included from ../os/auth.c:41:0:
../os/osdep.h:55:23: fatal error: X11/Xdmcp.h: No such file or directory
compilation terminated.
```

The configuration step succeeded.

Full build log: http://build.gnome.org/continuous/buildmaster/builds/2017/11/29/47/build/log-xorg-xserver.txt