configure: Detect uuid on MacOSX (fixes compile failure)

Commit 7791dba3ec broke compilation on MacOSX, because it introduced
a new include of util.h. On MacOSX this includes pwd.h which in turn
includes the system uuid/uuid.h, which causes a compile failure if
QEMU was configured without CONFIG_UUID due to a conflict between
the system header and our fallback versions:
  block/vdi.c:124:20: error: static declaration of 'uuid_generate'
  follows non-static declaration
  static inline void uuid_generate(uuid_t out)
                     ^
  /usr/include/uuid/uuid.h:63:6: note: previous declaration is here
  void uuid_generate(uuid_t out);
       ^

Fix this breakage by improving configure's check for uuid to work on
MacOSX (where there is no need to link in a separate libuuid).

Note that if the user explicitly runs configure with '--disable-uuid'
on MacOSX then QEMU will fail to compile.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1368563799-22755-1-git-send-email-peter.maydell@linaro.org
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
