configure: Detect uuid on MacOSX (fixes compile failure)

Commit 7791dba3 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: NPeter Maydell <peter.maydell@linaro.org>
Message-id: 1368563799-22755-1-git-send-email-peter.maydell@linaro.org
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

configure

@@ -1948,6 +1948,8 @@ fi
 
 ##########################################
 # uuid_generate() probe, used for vdi block driver
+# Note that on some systems (notably MacOSX) no extra library
+# need be linked to get the uuid functions.
 if test "$uuid" != "no" ; then
   uuid_libs="-luuid"
   cat > $TMPC << EOF
@@ -1959,7 +1961,9 @@ int main(void)
     return 0;
 }
 EOF
-  if compile_prog "" "$uuid_libs" ; then
+  if compile_prog "" "" ; then
+    uuid="yes"
+  elif compile_prog "" "$uuid_libs" ; then
     uuid="yes"
     libs_softmmu="$uuid_libs $libs_softmmu"
     libs_tools="$uuid_libs $libs_tools"