Make dbus optional

Use 'make CC="cc -DNO_DBUS"' to build without dbus
Set the default thread stack size

POSIX does not specify the stack size of a thread created with
pthread_create(). tox needs a lot of stack, more than musl libc's
default stack size of 80KB, but less than glibc's 8MB. It seems that
128KB is enough.
Please merge with the previous commit
Merge pull request #193 from doughdemon/nodbus

Make dbus optional
Use 256KiB as default stack size for new thread
Merge pull request #192 from doughdemon/stacksize

Set the default thread stack size
Added userdata to av callbacks

Added userdata argument to callback_av_video and callback_av_audio to match toxcore changes.
Merge pull request #199 from blucoat/master

Added userdata to av callbacks
Add support for pasting files in xlib
Fixed indentation error
Fix possible double-paste bug

Fixed a possible bug where both text and files would be sent if the
owner of the selection allowed for both in xlib.
Add support for copy/pasting inline images in Xlib

Images can be pasted from the clipboard if the owner can provide them in
an image/png format.  No support for incremental pasting yet so large
images do not work.  There seems to be a problem with RGB/BGR encoding
but it only affects the preview, and shows up fine in other clients
(tested with venom).
Handle escaped special characters in filenames

uTox now properly supports percent-escaped URIs from dragging and
dropping and from pasting.
Merge branch 'master' of https://github.com/blucoat/uTox
fix minor issue
support for dragging files (win32)
testing..
change for core filetransfer change
fix
Fixed decoding of RGB PNGs

This changes code in both xlib and shared with win32, so it's possible
that this broke something on that side as well.  I'm certain that the
corrections made to png.c are correct though.