Preparation for transition to libc++.

Move the dependency on libc++ to its own makefile so that we can switch in a
single place between libc++ and stlport.

Change-Id: Ie61e7d054dcd049e36d5e7298c27d8a4abe6edf7
diff --git a/runtime/Android.mk b/runtime/Android.mk
index 9df69f0..d433fd5 100644
--- a/runtime/Android.mk
+++ b/runtime/Android.mk
@@ -344,9 +344,6 @@
   art_clang := $(3)
 
   include $(CLEAR_VARS)
-  ifeq ($$(art_target_or_host),target)
-    include external/stlport/libstlport.mk
-  endif
   LOCAL_CPP_EXTENSION := $(ART_CPP_EXTENSION)
   ifeq ($$(art_ndebug_or_debug),ndebug)
     LOCAL_MODULE := libart
@@ -366,6 +363,8 @@
     LOCAL_IS_HOST_MODULE := true
   endif
 
+  include art/build/Android.libcxx.mk
+
   GENERATED_SRC_DIR := $$(call local-generated-sources-dir)
   ENUM_OPERATOR_OUT_CC_FILES := $$(patsubst %.h,%_operator_out.cc,$$(LIBART_ENUM_OPERATOR_OUT_HEADER_FILES))
   ENUM_OPERATOR_OUT_GEN := $$(addprefix $$(GENERATED_SRC_DIR)/,$$(ENUM_OPERATOR_OUT_CC_FILES))