Support stl variants when LOCAL_NDK_VERSION is set.

LOCAL_NDK_STL_VARIANT can be set to system, stlport_static, stlport_shared
or gnustl_static. It defaults to system.

Change-Id: I2840d30ff732a00bc0ea70eab8a8179aea0abbdf
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 0d2e215..f947bf6 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -92,6 +92,7 @@
 LOCAL_CERTIFICATE:=
 LOCAL_SDK_VERSION:=
 LOCAL_NDK_VERSION:=
+LOCAL_NDK_STL_VARIANT:=
 LOCAL_NO_EMMA_INSTRUMENT:=
 LOCAL_NO_EMMA_COMPILE:=
 LOCAL_PROGUARD_ENABLED:= # '',optonly,full,custom,disabled