fix Android standalone toolchain build

add a check for cpu-features.h and rework some of the ifdef's around
android + neon. for android builds with cpu-features enabled the
*_neon.c files will still need to be flagged correctly (with e.g.,
.c.neon in Android.mk) to properly build them.

BUG=webp:353

Change-Id: I905ce305af0a204e560b915d8665093a3edaceb9

Android.mk

@@ -25,6 +25,7 @@ ifneq ($(findstring armeabi-v7a, $(TARGET_ARCH_ABI)),)
   # specifically.
   NEON := c.neon
   USE_CPUFEATURES := yes
+  WEBP_CFLAGS += -DHAVE_CPU_FEATURES_H
 else
   NEON := c
 endif