Add host protoc script back (#159157)

Following comment from https://github.com/pytorch/pytorch/pull/158475#issuecomment-3116518904

Also this is a fake issue as protoc is dead anyways: https://github.com/pytorch/pytorch/issues/159156

Also also, macos cross compilation is not something that is tested :/ But I guess we're ok with that given how niche it it...
Pull Request resolved: https://github.com/pytorch/pytorch/pull/159157
Approved by: https://github.com/janeyx99

scripts/build_host_protoc.sh

@@ -0,0 +1,59 @@
+#!/bin/bash
+##############################################################################
+# Build script to build the protoc compiler for the host platform.
+##############################################################################
+# This script builds the protoc compiler for the host platform, which is needed
+# for any cross-compilation as we will need to convert the protobuf source
+# files to cc files.
+#
+# --other-flags accepts flags that should be passed to cmake. Optional.
+#
+# After the execution of the file, one should be able to find the host protoc
+# binary at build_host_protoc/bin/protoc.
+
+set -e
+
+CAFFE2_ROOT="$( cd "$(dirname -- "$0")"/.. ; pwd -P)"
+BUILD_ROOT=${BUILD_ROOT:-"$CAFFE2_ROOT/build_host_protoc"}
+mkdir -p $BUILD_ROOT/build
+cd $BUILD_ROOT/build
+
+CMAKE_ARGS=()
+CMAKE_ARGS+=("-DCMAKE_INSTALL_PREFIX=$BUILD_ROOT")
+CMAKE_ARGS+=("-Dprotobuf_BUILD_TESTS=OFF")
+
+# If Ninja is installed, prefer it to Make
+if [ -x "$(command -v ninja)" ]; then
+  CMAKE_ARGS+=("-GNinja")
+fi
+
+while true; do
+    case "$1" in
+        --other-flags)
+            shift;
+            CMAKE_ARGS+=("$@")
+            break ;;
+        "")
+            break ;;
+        *)
+            echo "Unknown option passed as argument: $1"
+            break ;;
+    esac
+done
+
+# Use ccache if available (this path is where Homebrew installs ccache symlinks)
+if [ "$(uname)" == 'Darwin' ] && [ -d /usr/local/opt/ccache/libexec ]; then
+  CMAKE_ARGS+=("-DCMAKE_C_COMPILER=/usr/local/opt/ccache/libexec/gcc")
+  CMAKE_ARGS+=("-DCMAKE_CXX_COMPILER=/usr/local/opt/ccache/libexec/g++")
+fi
+
+cmake "$CAFFE2_ROOT/third_party/protobuf/cmake" ${CMAKE_ARGS[@]}
+
+if [ -z "$MAX_JOBS" ]; then
+  if [ "$(uname)" == 'Darwin' ]; then
+    MAX_JOBS=$(sysctl -n hw.ncpu)
+  else
+    MAX_JOBS=$(nproc)
+  fi
+fi
+cmake --build . -- "-j${MAX_JOBS}" install