[android] Enable compile with clang

Glue with libBlocksRuntime and compile with
minimum API 23 which works with current HF
ptrace backend.

Use 'ANDROID_CLANG=true' flag to enable
clang builds (default off)

Signed-off-by: Anestis Bechtsoudis <anestis@census-labs.com>

# third_party/android/scripts/compile-libBlocksRuntime.sh third_party/android/libBlocksRuntime arm
# make android ANDROID_CLANG=true

Makefile

diff --git a/Makefile b/Makefile
index 9ac4155..4af53dd 100644
--- a/Makefile
+++ b/Makefile
@@ -145,14 +145,32 @@
 
 # Control Android builds
 ANDROID_DEBUG_ENABLED ?= false
+ANDROID_CLANG         ?= false
 ANDROID_APP_ABI       ?= armeabi-v7a
-ANDROID_API           ?= android-21
-ANDROID_NDK_TOOLCHAIN ?=
 NDK_BUILD_ARGS :=
 ifeq ($(ANDROID_DEBUG_ENABLED),true)
     NDK_BUILD_ARGS += V=1 NDK_DEBUG=1 APP_OPTIM=debug
 endif
 
+ifeq ($(ANDROID_CLANG),true)
+  # clang works only for APIs >= 23, so default to it if not set
+  ANDROID_API ?= android-23
+  ifeq ($(ANDROID_APP_ABI),armeabi-v7a)
+    ANDROID_NDK_TOOLCHAIN ?= arm-linux-androideabi-clang3.6
+  else ifeq ($(ANDROID_APP_ABI),x86)
+    ANDROID_NDK_TOOLCHAIN ?= x86-clang3.6
+  else ifeq ($(ANDROID_APP_ABI),arm64-v8a)
+    ANDROID_NDK_TOOLCHAIN ?= aarch64-linux-android-clang3.6
+  else ifeq ($(ANDROID_APP_ABI),x86_64)
+    ANDROID_NDK_TOOLCHAIN ?= x86_64-clang3.6
+  else
+    $(error Unsuported / Unknown APP_API '$(ANDROID_APP_ABI)')
+  endif
+else
+  ANDROID_API           ?= android-21
+  ANDROID_NDK_TOOLCHAIN ?=
+endif
+
 SUBDIR_ROOTS := linux mac posix libraries
 DIRS := . $(shell find $(SUBDIR_ROOTS) -type d)
 CLEAN_PATTERNS := *.o *~ core *.a *.dSYM *.la *.so *.dylib