configure: Define OS_OBJECT_USE_OBJC=0 for MacOSX builds

configure: Define OS_OBJECT_USE_OBJC=0 for MacOSX builds

MacOSX 10.8 ("Mountain Lion") defaults to trying to use automated
reference counting on certain objects.  This means that the system
header files will use some Objective C syntax constructs even when
compiling pure C, which confuses mainline gcc. Suppress this by
setting OS_OBJECT_USE_OBJC=0. This avoids a compile error like this:

In file included from
/System/Library/Frameworks/Foundation.framework/Headers/NSObject.h:5:0,
                 from /usr/include/os/object.h:74,
                 from /usr/include/dispatch/dispatch.h:48,
                 from /System/Library/Frameworks/IOKit.framework/Headers/IOKitLib.h:56,
                 from block/raw-posix.c:35:
/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:409:1: error: stray ‘@’ in program
[with a large number of further run-on errors]

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>