When SDL support is disabled, there is no way to build QEMU without
Cocoa support on MacOS X. This patch adds '--disable-cocoa' switch and
allows to build QEMU without both SDL and Cocoa frontends.
Signed-off-by: NPavel Borzenkov <pavel.borzenkov@gmail.com>
[AF: Adapt help output]
Signed-off-by: NAndreas Färber <andreas.faerber@web.de>