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/build/Android.executable.mk b/build/Android.executable.mk
index 27d687c..6aa1c18 100644
--- a/build/Android.executable.mk
+++ b/build/Android.executable.mk
@@ -52,10 +52,6 @@
   art_multilib := $(7)
 
   include $(CLEAR_VARS)
-  ifeq ($$(art_target_or_host),target)
-    include external/stlport/libstlport.mk
-  endif
-
   LOCAL_CPP_EXTENSION := $(ART_CPP_EXTENSION)
   LOCAL_MODULE_TAGS := optional
   LOCAL_SRC_FILES := $$(art_source)
@@ -104,9 +100,12 @@
   endif
 
   ifeq ($$(art_target_or_host),target)
+    include art/build/Android.libcxx.mk
     include $(BUILD_EXECUTABLE)
     ART_TARGET_EXECUTABLES := $(ART_TARGET_EXECUTABLES) $(TARGET_OUT_EXECUTABLES)/$$(LOCAL_MODULE)
   else # host
+    LOCAL_IS_HOST_MODULE := true
+    include art/build/Android.libcxx.mk
     include $(BUILD_HOST_EXECUTABLE)
     ART_HOST_EXECUTABLES := $(ART_HOST_EXECUTABLES) $(HOST_OUT_EXECUTABLES)/$$(LOCAL_MODULE)
   endif