Please describe the problem:
gdm-2.26.1 uses a new "interpreted" xauth family that is only seems to be available on very new x11. It's not checked in ./configure, and it would be more portable to have some way to make this use optional.

Steps to reproduce:
1. Use OS X 10.4/apple x11
2. Use fink to supply all dependencies at the level of GNOME2.26
3. Unpack gdm-2.26.1
4. ./configure with various options (doesn't seem to matter, since the code in question is never not-used)
5. make


Actual results:
gcc -DHAVE_CONFIG_H -I. -I..  -I. -I.. -I../common -DAUTHDIR=\"/sw/var/gdm\" -DBINDIR=\"/sw/bin\" -DDATADIR=\"/sw/share\" -DDMCONFDIR=\"/sw/etc/dm\" -DGDMCONFDIR=\"/sw/etc/gdm\" -DLIBDIR=\"/sw/lib\" -DLIBEXECDIR=\"/sw/lib/gdm\" -DLOGDIR=\"/sw/var/log/gdm\" -DSBINDIR=\"/sw/sbin\" -DGNOMELOCALEDIR=\""/sw/share/locale"\" -DGDM_XAUTH_DIR=\"/sw/var/run/gdm\" -DGDM_SESSION_DEFAULT_PATH=\"/usr/local/bin:/usr/bin:/bin\"  -I/sw/include/dbus-1.0 -I/sw/lib/dbus-1.0/include -I/sw/include/glib-2.0 -I/sw/lib/glib-2.0/include -I/usr/X11R6/include -Wall -Wmissing-prototypes  -DG_ENABLE_DEBUG -DLANG_CONFIG_FILE=\"/sw/etc/sysconfig/i18n\"  -I/sw/include  -Wall         -Wchar-subscripts -Wmissing-declarations -Wmissing-prototypes         -Wnested-externs -Wpointer-arith         -Wcast-align -Wsign-compare -Wp,-D_FORTIFY_SOURCE=2         -g -O2 -Wno-strict-aliasing -Wno-sign-compare -c gdm-slave.c
gdm-slave.c: In function 'gdm_slave_connect_to_x11_display':
gdm-slave.c:398: error: 'FamilyServerInterpreted' undeclared (first use in this function)
gdm-slave.c:398: error: (Each undeclared identifier is reported only once
gdm-slave.c:398: error: for each function it appears in.)
gdm-slave.c:401: error: 'XServerInterpretedAddress' undeclared (first use in this function)
gdm-slave.c:401: error: parse error before 'si_entries'
gdm-slave.c:409: error: 'si_entries' undeclared (first use in this function)
make[3]: *** [gdm-slave.o] Error 1
make[2]: *** [all] Error 2
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2


Expected results:
Compiles successfully

Does this happen every time?
yes

Other information:
manpage and Xlib.h confirm that FamilyServerInterpreted is not a supported host family.

Neither OS X 10.5 (latest publicly available version, with apple's x11) nor xquartz (more advanced x11 for OS X, approximately the development branch for apple's x11) appear to have this family either.