Only set USE_ANDROID_NDK_CPU_FEATURES if it's not already been explicitly set

R=djsollen@google.com, reed@google.com
BUG=skia:

Author: george@mozilla.com

Review URL: https://codereview.chromium.org/189263015

git-svn-id: http://skia.googlecode.com/svn/trunk@13750 2bbb7eff-a529-9590-31e7-b0007b416f81

src/core/SkUtilsArm.cpp

@@ -20,10 +20,12 @@
 // cpu-features helper library to detect NEON at runtime. See
 // http://crbug.com/164154 to see why this is needed in Chromium
 // for Android.
-#if defined(SK_BUILD_FOR_ANDROID)
-#  define USE_ANDROID_NDK_CPU_FEATURES 1
-#else
-#  define USE_ANDROID_NDK_CPU_FEATURES 0
+#if !defined(USE_ANDROID_NDK_CPU_FEATURES)
+#  if defined(SK_BUILD_FOR_ANDROID)
+#    define USE_ANDROID_NDK_CPU_FEATURES 1
+#  else
+#    define USE_ANDROID_NDK_CPU_FEATURES 0
+#  endif
 #endif
 
 #if USE_ANDROID_NDK_CPU_FEATURES