android: build with libcxx on android lollipop

On Lollipop, apparently stlport is gone and libcxx must be used instead.
We still support stlport when building on earlier android releases.

Signed-off-by: Chih-Wei Huang <cwhuang@linux.org.tw>
Reviewed-by: Eric Anholt <eric@anholt.net>
diff --git a/Android.common.mk b/Android.common.mk
index 43766bf..d662d60 100644
--- a/Android.common.mk
+++ b/Android.common.mk
@@ -77,6 +77,7 @@
 endif
 
 LOCAL_CPPFLAGS += \
+	$(if $(filter true,$(MESA_LOLLIPOP_BUILD)),-D_USING_LIBCXX) \
 	-Wno-error=non-virtual-dtor \
 	-Wno-non-virtual-dtor