This is what the build log says:
----------------------
Making all in hw
Making all in xwin
Making all in vfb
LINK Xvfb.exe
gcc: unrecognized option `-rdynamic'
../../mi/.libs/libmi.a(miexpose.o): In function `miPaintWindow':
/usr/src/xorg-server-1.7.3-1/src/xorg-server-1.7.3/mi/miexpose.c:555:
undefined reference to `_IsFramedWindow'
/usr/src/xorg-server-1.7.3-1/src/xorg-server-1.7.3/mi/miexpose.c:556:
undefined reference to `_RootlessStartDrawing'
/usr/src/xorg-server-1.7.3-1/src/xorg-server-1.7.3/mi/miexpose.c:557:
undefined reference to `_RootlessDamageRegion'
/usr/src/xorg-server-1.7.3-1/src/xorg-server-1.7.3/mi/miexpose.c:562:
undefined reference to `_RootlessSetPixmapOfAncestors'
collect2: ld returned 1 exit status
make[3]: *** [Xvfb.exe] Error 1
make[2]: *** [all] Error 2
make[1]: *** [all-recursive] Error 1
make: *** [all-recursive] Error 1
----------------------
I can see those functions in miext/rootless so it looks like
librootless is somehow not linked in?