Support using a different compiler for Objective-C files

Support using a different compiler for Objective-C files

MacOSX 10.8 ("Mountain Lion") requires us to compile our one
Objective-C source file with clang even if the rest of QEMU
requires a real gcc, because the system headers we use make
use of Apple's "Blocks" extension to C/ObjC, and mainline
gcc doesn't support that. Since we only need to use a true
gcc for the parts of QEMU that use the fixed-register
env variable, we can simply use clang to build the ObjC
file: it will link to the gcc-built objects with no problems.

Add the necessary support for an OBJCC variable in the
makefile and configure machinery; we default to clang
if we have it, otherwise whatever CC is (since gcc
might be the Apple gcc which does support Blocks).

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