use gcc-4.2 on OS X 10.5, or clang on Xcode 4

vm/Config.macosx

@@ -21,3 +21,17 @@ LINKER = $(CPP) $(CFLAGS) -dynamiclib -single_module \
 	$(LIBS) $(LIBPATH) -o
 
 LINK_WITH_ENGINE = -lfactor
+
+LEOPARD_P := $(shell [[ `uname -r | sed -E -e 's/^([0-9]+).*/\1/'` -le 9 ]]; echo $$?)
+XCODE_4_P := $(shell [[ `xcodebuild -version | sed -E -ne 's/^Xcode ([0-9]+).*/\1/p'` -ge 4 ]]; echo $$?)
+
+ifeq ($(LEOPARD_P),0)
+	CC = gcc-4.2
+	CPP = g++-4.2
+endif
+
+ifeq ($(XCODE_4_P),0)
+	CC = clang
+	CPP = clang++
+endif
+