Restrict building Subzero to Marshmallow and up.

Subzero requires full C++11 support, which isn't available on Android
versions prior to Marshmallow.

Change-Id: Icf09a51b525a1503f72441c969a9e364306096bd
Reviewed-on: https://swiftshader-review.googlesource.com/11488
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>

Android.mk

diff --git a/Android.mk b/Android.mk
index b4d82e3..a89cdf0 100644
--- a/Android.mk
+++ b/Android.mk
@@ -14,8 +14,22 @@
 # limitations under the License.
 #
 
-LOCAL_PATH:= $(call my-dir)
+LOCAL_PATH := $(call my-dir)
 
+# Use Subzero as the Reactor JIT back-end on ARM, else LLVM.
+ifeq ($(TARGET_ARCH),$(filter $(TARGET_ARCH),arm))
+use_subzero := true
+endif
+
+# Subzero requires full C++11 support, which is available from Marshmallow and up.
+ifdef use_subzero
+ifeq ($(shell test $(PLATFORM_SDK_VERSION) -lt 23 && echo PreMarshmallow),PreMarshmallow)
+unsupported_build := true
+endif
+endif
+
+ifndef unsupported_build
 ifeq ($(TARGET_ARCH),$(filter $(TARGET_ARCH),x86 x86_64 arm))
 include $(call all-makefiles-under,$(LOCAL_PATH))
 endif
+endif
\ No newline at end of file

src/Android.mk

diff --git a/src/Android.mk b/src/Android.mk
index c067be6..7f1efa2 100644
--- a/src/Android.mk
+++ b/src/Android.mk
@@ -1,10 +1,5 @@
 LOCAL_PATH:= $(call my-dir)
 
-# Use Subzero as the Reactor JIT back-end on ARM, else LLVM.
-ifeq ($(TARGET_ARCH),$(filter $(TARGET_ARCH),arm))
-use_subzero := true
-endif
-
 COMMON_C_INCLUDES += \
 	bionic \
 	$(LOCAL_PATH)/../include \