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
fix minor issue
support for dragging files (win32)
testing..
change for core filetransfer change
fix
Audiobot