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

build.gradle

@@ -84,6 +84,7 @@ model {
       // Check for NEON usage.
       if (getTargetPlatform() == "arm") {
         NEON = "c.neon"
+        cCompiler.define "HAVE_CPU_FEATURES_H"
       } else {
         NEON = "c"
       }