Support WatchOS build

cmake/ProtoBuf.cmake

@@ -28,8 +28,15 @@ function(custom_protobuf_find)
   set(Protobuf_FOUND TRUE PARENT_SCOPE)
 endfunction()
 
-if (ANDROID OR IOS OR WIN32)
+if (WIN32)
   custom_protobuf_find()
+elseif (ANDROID OR IOS)
+  custom_protobuf_find()
+  # For Androd or iOS, we won't need to build the libprotoc and protoc binaries
+  # because we will use the host protoc to build the proto files.
+  set_target_properties(
+      libprotoc protoc PROPERTIES
+      EXCLUDE_FROM_ALL 1 EXCLUDE_FROM_DEFAULT_BUILD 1)
 else()
   find_package( Protobuf )
   if ( NOT (Protobuf_FOUND OR PROTOBUF_FOUND) )

scripts/build_ios.sh

@@ -22,11 +22,17 @@ $CAFFE2_ROOT/scripts/build_host_protoc.sh || exit 1
 echo "Building caffe2"
 cd $BUILD_ROOT
 
+if [ -z ${IOS_PLATFORM+x} ]; then
+    # IOS_PLATFORM is not set, in which case we will default to OS, which
+    # builds iOS.
+    IOS_PLATFORM=OS
+fi
+
 cmake .. \
     -DCMAKE_TOOLCHAIN_FILE=$CAFFE2_ROOT/third_party/ios-cmake/toolchain/iOS.cmake\
     -DCMAKE_INSTALL_PREFIX=../install \
     -DCMAKE_BUILD_TYPE=Release \
-    -DIOS_PLATFORM=OS \
+    -DIOS_PLATFORM=${IOS_PLATFORM} \
     -DUSE_CUDA=OFF \
     -DBUILD_TEST=OFF \
     -DBUILD_BINARY=OFF \