address comments

src/csharp/experimental/build_native_ext_for_android.sh

@@ -27,7 +27,7 @@ cmake ../.. \
   -DCMAKE_SYSTEM_NAME=Android \
   -DCMAKE_SYSTEM_VERSION=15 \
   -DCMAKE_ANDROID_ARCH_ABI=armeabi-v7a \
-  -DCMAKE_ANDROID_NDK=${ANDROID_NDK_PATH} \
+  -DCMAKE_ANDROID_NDK="${ANDROID_NDK_PATH}" \
   -DCMAKE_ANDROID_STL_TYPE=c++_static \
   -DRUN_HAVE_POSIX_REGEX=0 \
   -DRUN_HAVE_STD_REGEX=0 \