To prevent warnings passing by unnoticed, I compile with -Werror in my CFLAGS.  Since some time this results in the following failure:

make[4]: Entering directory '/home/ben/Sources/openchrome/src'
/bin/sh ../libtool  --tag=CC   --mode=compile gcc -DHAVE_CONFIG_H -I. -I..   -fvisibility=hidden -I/usr/include/xorg -I/usr/include/pixman-1 -I/usr/include/X11/dri -I/usr/include/libdrm  -fvisibility=hidden -I/usr/include/xorg -I/usr/include/pixman-1 -I/usr/include/X11/dri -I/usr/include/libdrm  -I/usr/include/libdrm -I/usr/include/X11/dri -g -O2 -Werror -MT via_ums.lo -MD -MP -MF .deps/via_ums.Tpo -c -o via_ums.lo via_ums.c
libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I.. -fvisibility=hidden -I/usr/include/xorg -I/usr/include/pixman-1 -I/usr/include/X11/dri -I/usr/include/libdrm -fvisibility=hidden -I/usr/include/xorg -I/usr/include/pixman-1 -I/usr/include/X11/dri -I/usr/include/libdrm -I/usr/include/libdrm -I/usr/include/X11/dri -g -O2 -Werror -MT via_ums.lo -MD -MP -MF .deps/via_ums.Tpo -c via_ums.c  -fPIC -DPIC -o .libs/via_ums.o
via_ums.c: In function ‘umsCreate’:
via_ums.c:745:11: error: implicit declaration of function ‘xf86InitFBManager’ [-Werror=implicit-function-declaration]
     ret = xf86InitFBManager(pScreen, &AvailFBArea);
           ^~~~~~~~~~~~~~~~~
via_ums.c:756:9: error: implicit declaration of function ‘xf86InitFBManagerLinear’ [-Werror=implicit-function-declaration]
         xf86InitFBManagerLinear(pScreen, offset, size);
         ^~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
Makefile:593: recipe for target 'via_ums.lo' failed
make[4]: *** [via_ums.lo] Error 1

It seems that some header file, in which xf86InitFBManager() and xf86InitFBManagerLinear() are defined, has gone missing or are mistakenly not included.  But I have no idea where to look for the definition of these functions.  Googling doesn't make me any the wiser.