When I build emacs against gtk3, it is unusable.  Here are the symptoms 
when the resulting emacs is started in an xterm window:

$ ./emacs -Q&
[1] 3344

(emacs:3344): GLib-WARNING **: In call to g_spawn_sync(), exit status of 
a child process was requested but SIGCHLD action was set to SIG_IGN and 
ECHILD was received by waitpid(), so exit status can't be returned. This 
is a bug in the program calling g_spawn_sync(); either don't request the 
exit status, or don't set the SIGCHLD action.

** (emacs:3344): WARNING **: Abnormal program termination spawning 
command line `dbus-launch --autolaunch=0b8f184fe6d82872ee8db8724ecfdb90 
--binary-syntax --close-stderr':

(emacs:3344): Pango-WARNING **: No such file or directory

A few seconds later, emacs dies (the window disappears and the process 
is gone, with no error messages), and two dbus processes remain:

$ ps | grep dbus
      5188       1    3344       5188    7 1002 19:07:31 
/usr/bin/dbus-launch
      6452       1    6452       6452    ? 1002 19:07:31 
/usr/bin/dbus-daemon

If I start emacs again without killing the dbus processes, I don't get 
the first two warnings but I still get the third.  Again, emacs dies 
after a few seconds.

I think the pango warning is Cygwin specific, but the rest of it might 
not be.  Similar symptoms were reported on Fedora:

   https://bugzilla.redhat.com/show_bug.cgi?id=654027

But there are some differences, so I thought I should report it here 
just in case part of the problem is Cygwin specific.  My cygcheck output 
is attached but probably not relevant.

Ken