Silence Ninja build errors when the Android NDK is missing.

Rather than fail the build, we just skip the Android builds when
the NDK is missing now.

Change-Id: I9d4ff5e3dddf5541de2d498be4d3da1bfa70f7b3
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/776696
Commit-Queue: Ben Wagner <bungeman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
diff --git a/ninja/android b/ninja/android
index 4420d22..8cfe4e8 100644
--- a/ninja/android
+++ b/ninja/android
@@ -16,3 +16,5 @@
               else touch $out; fi
     description = run $in
 include ninja/common
+
+disabled = ! test -d $ndk
diff --git a/ninja/android-arm b/ninja/android-arm
index c9fea90..8c9c31c 100644
--- a/ninja/android-arm
+++ b/ninja/android-arm
@@ -16,3 +16,5 @@
               else touch $out; fi
     description = run $in
 include ninja/common
+
+disabled = ! test -d $ndk