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>

configure

@@ -543,6 +543,13 @@ fi
 : ${python=${PYTHON-python}}
 : ${smbd=${SMBD-/usr/sbin/smbd}}
 
+# Default objcc to clang if available, otherwise use CC
+if has clang; then
+  objcc=clang
+else
+  objcc="$cc"
+fi
+
 if test "$mingw32" = "yes" ; then
   EXESUF=".exe"
   QEMU_CFLAGS="-DWIN32_LEAN_AND_MEAN -DWINVER=0x501 $QEMU_CFLAGS"
@@ -586,6 +593,8 @@ for opt do
   ;;
   --host-cc=*) host_cc="$optarg"
   ;;
+  --objcc=*) objcc="$optarg"
+  ;;
   --make=*) make="$optarg"
   ;;
   --install=*) install="$optarg"
@@ -1028,6 +1037,7 @@ echo "  --cross-prefix=PREFIX    use PREFIX for compile tools [$cross_prefix]"
 echo "  --cc=CC                  use C compiler CC [$cc]"
 echo "  --host-cc=CC             use C compiler CC [$host_cc] for code run at"
 echo "                           build time"
+echo "  --objcc=OBJCC            use Objective-C compiler OBJCC [$objcc]"
 echo "  --extra-cflags=CFLAGS    append extra C compiler flags QEMU_CFLAGS"
 echo "  --extra-ldflags=LDFLAGS  append extra linker flags LDFLAGS"
 echo "  --make=MAKE              use specified make [$make]"
@@ -3058,6 +3068,7 @@ fi
 echo "Source path       $source_path"
 echo "C compiler        $cc"
 echo "Host C compiler   $host_cc"
+echo "Objective-C compiler $objcc"
 echo "CFLAGS            $CFLAGS"
 echo "QEMU_CFLAGS       $QEMU_CFLAGS"
 echo "LDFLAGS           $LDFLAGS"
@@ -3521,6 +3532,7 @@ echo "PYTHON=$python" >> $config_host_mak
 echo "CC=$cc" >> $config_host_mak
 echo "CC_I386=$cc_i386" >> $config_host_mak
 echo "HOST_CC=$host_cc" >> $config_host_mak
+echo "OBJCC=$objcc" >> $config_host_mak
 echo "AR=$ar" >> $config_host_mak
 echo "OBJCOPY=$objcopy" >> $config_host_mak
 echo "LD=$ld" >> $config_host_mak

rules.mak

@@ -29,7 +29,7 @@ endif
 	$(call quiet-command,$(CC) $(QEMU_INCLUDES) $(QEMU_CFLAGS) $(QEMU_DGFLAGS) $(CFLAGS) -c -o $@ $<,"  AS    $(TARGET_DIR)$@")
 
 %.o: %.m
-	$(call quiet-command,$(CC) $(QEMU_INCLUDES) $(QEMU_CFLAGS) $(QEMU_DGFLAGS) $(CFLAGS) -c -o $@ $<,"  OBJC  $(TARGET_DIR)$@")
+	$(call quiet-command,$(OBJCC) $(QEMU_INCLUDES) $(QEMU_CFLAGS) $(QEMU_DGFLAGS) $(CFLAGS) -c -o $@ $<,"  OBJC  $(TARGET_DIR)$@")
 
 LINK = $(call quiet-command,$(CC) $(QEMU_CFLAGS) $(CFLAGS) $(LDFLAGS) -o $@ $(sort $(1)) $(LIBS),"  LINK  $(TARGET_DIR)$@")