gb_OBJCXXFLAGS requires -lobjc

...as it causes at least recent Clang trunk (towards 3.7) to emit

  .cfi_personality 155, ___objc_personality_v0

where __objc_personality_v0 is exported from /usr/lib/libobjc.A.dylib

Change-Id: If4ccf15b2c3167185ba50d78ecf4379e02814933

toolkit/Library_tk.mk

@@ -119,6 +119,9 @@ ifeq ($(OS),MACOSX)
 $(eval $(call gb_Library_add_cxxflags,tk,\
 	$(gb_OBJCXXFLAGS) \
 ))
+$(eval $(call gb_Library_add_libs,tk,\
+	-lobjc \
+))
 endif
 
 ifeq ($(OS),IOS)