Fix build on OSX, eggsmclient-osx.c must be compied in objective-c mode.

revert change on ce3af909b5cb5c7ce0a67b5ac4d7a81b457950b7
@robc, why that commit is named `Fix build on non-OSX`

`frontend/gtkmm/src/eggsmclient-osx.c` must be compied in objective-c mode.
However, the `AM_CPFLAGS` apply on it, so `CFLAGS` is used as hack for .c.o.
fix for build on non-DBUS
fix memory leak when play sound.

I find this issue for very long time.
However, I didn't find a good way to fix it.
Now, I use a bad way to do this.

As QTKIT/QUICKTIME is depreceated in OSX, I change to NSSound.
I keep an dictionary for the sound file and NSSound object to avoid
new instance for NSSound every time.
Merge pull request #36 from liudongmiao/master

Patch for Mac OS X again