The workaround is to copy the copy of small-sets.h that exists in the source/folks/folks directory to the install/include/folks/, e.g.:

cp gnome/source/folks/folks/small-set.h gnome/install/include/folks/

However, in the current state, the failure to copy the small-set.h file causes jhbuild to fail when building empathy, e.g.:

make[3]: Entering directory `/home/waltman/Downloads/gnome/source/empathy/libempathy'
  CC     action-chain.lo
  CC     empathy-account-settings.lo
  CC     empathy-auth-factory.lo
  CC     empathy-camera-monitor.lo
  CC     empathy-chatroom-manager.lo
  CC     empathy-chatroom.lo
  CC     empathy-client-factory.lo
  CC     empathy-connection-managers.lo
  CC     empathy-connection-aggregator.lo
  CC     empathy-contact-groups.lo
  CC     empathy-contact.lo
  CC     empathy-debug.lo
  CC     empathy-ft-factory.lo
  CC     empathy-ft-handler.lo
In file included from empathy-utils.h:33:0,
                 from empathy-connection-managers.c:24:
/home/waltman/Downloads/gnome/install/include/folks/folks-telepathy.h:11:29: fatal error: folks/small-set.h: No such file or directory
compilation terminated.
In file included from empathy-utils.h:33:0,
                 from empathy-account-settings.c:27:
/home/waltman/Downloads/gnome/install/include/folks/folks-telepathy.h:11:29: fatal error: folks/small-set.h: No such file or directory
compilation terminated.
In file included from empathy-utils.h:33:0,
                 from empathy-chatroom.c:25:
/home/waltman/Downloads/gnome/install/include/folks/folks-telepathy.h:11:29: fatal error: folks/small-set.h: No such file or directory
compilation terminated.
In file included from empathy-utils.h:33:0,
                 from empathy-auth-factory.c:28:
/home/waltman/Downloads/gnome/install/include/folks/folks-telepathy.h:11:29: fatal error: folks/small-set.h: No such file or directory
compilation terminated.
In file included from empathy-utils.h:33:0,
                 from empathy-contact-groups.c:28:
/home/waltman/Downloads/gnome/install/include/folks/folks-telepathy.h:11:29: fatal error: folks/small-set.h: No such file or directory
compilation terminated.
  CC     empathy-presence-manager.lo
In file included from empathy-utils.h:33:0,
                 from empathy-client-factory.c:26:
/home/waltman/Downloads/gnome/install/include/folks/folks-telepathy.h:11:29: fatal error: folks/small-set.h: No such file or directory
compilation terminated.
In file included from empathy-utils.h:33:0,
                 from empathy-chatroom-manager.c:30:
/home/waltman/Downloads/gnome/install/include/folks/folks-telepathy.h:11:29: fatal error: folks/small-set.h: No such file or directory
compilation terminated.
  CC     empathy-individual-manager.lo
  CC     empathy-irc-network-manager.lo
In file included from empathy-utils.h:33:0,
                 from empathy-contact.c:29:
/home/waltman/Downloads/gnome/install/include/folks/folks-telepathy.h:11:29: fatal error: folks/small-set.h: No such file or directory
compilation terminated.
In file included from empathy-utils.h:33:0,
                 from empathy-ft-factory.c:28:
/home/waltman/Downloads/gnome/install/include/folks/folks-telepathy.h:11:29: fatal error: folks/small-set.h: No such file or directory
compilation terminated.
In file included from empathy-utils.h:33:0,
                 from empathy-ft-handler.c:30:
/home/waltman/Downloads/gnome/install/include/folks/folks-telepathy.h:11:29: fatal error: folks/small-set.h: No such file or directory
compilation terminated.
In file included from empathy-utils.h:33:0,
                 from empathy-irc-network-manager.c:26:
/home/waltman/Downloads/gnome/install/include/folks/folks-telepathy.h:11:29: fatal error: folks/small-set.h: No such file or directory
compilation terminated.
In file included from empathy-utils.h:33:0,
                 from empathy-presence-manager.c:24:
/home/waltman/Downloads/gnome/install/include/folks/folks-telepathy.h:11:29: fatal error: folks/small-set.h: No such file or directory
compilation terminated.
In file included from empathy-utils.h:33:0,
                 from empathy-individual-manager.c:26:
/home/waltman/Downloads/gnome/install/include/folks/folks-telepathy.h:11:29: fatal error: folks/small-set.h: No such file or directory
compilation terminated.
make[3]: *** [empathy-account-settings.lo] Error 1
make[3]: *** Waiting for unfinished jobs....
make[3]: *** [empathy-client-factory.lo] Error 1
make[3]: *** [empathy-chatroom.lo] Error 1
make[3]: *** [empathy-contact-groups.lo] Error 1
make[3]: *** [empathy-contact.lo] Error 1
make[3]: *** [empathy-connection-managers.lo] Error 1
make[3]: *** [empathy-auth-factory.lo] Error 1
make[3]: *** [empathy-ft-factory.lo] Error 1
make[3]: *** [empathy-chatroom-manager.lo] Error 1
make[3]: *** [empathy-ft-handler.lo] Error 1
make[3]: *** [empathy-irc-network-manager.lo] Error 1
make[3]: *** [empathy-presence-manager.lo] Error 1
make[3]: *** [empathy-individual-manager.lo] Error 1
make[3]: Leaving directory `/home/waltman/Downloads/gnome/source/empathy/libempathy'
make[2]: *** [all] Error 2
make[2]: Leaving directory `/home/waltman/Downloads/gnome/source/empathy/libempathy'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/home/waltman/Downloads/gnome/source/empathy'
make: *** [all] Error 2
*** Error during phase build of empathy: ########## Error running make -j 13 *** [102/197]

  [1] Rerun phase build
  [2] Ignore error and continue to install
  [3] Give up on module
  [4] Start shell
  [5] Reload configuration
  [6] Go to phase "wipe directory and start over"
  [7] Go to phase "configure"
  [8] Go to phase "clean"
  [9] Go to phase "distclean"
choice: