Fix for Android NDK r12

`android-ndk-r12/sources/android/support/Android.mk` now
conditionally compiles based on the `LIBCXX_FORCE_REBUILD`
environment variable. Add the variable to our `build.sh`
invocation in order to ensure we actually build the stuff we need.

build.sh

@@ -24,7 +24,7 @@ mkdir -p android_support
 cd android_support
 ln -sf $NDK/sources/android/support jni
 
-ndk-build -j$NCPU APP_ABI=$ARCH || exit 1
+ndk-build -j$NCPU APP_ABI=$ARCH LIBCXX_FORCE_REBUILD=true || exit 1
 cp -f obj/local/$ARCH/libandroid_support.a ../
 
 } || exit 1