Add support for user-specified protoc

Make it possible to specify your own protoc when building protobuf in
a submodule.

When cross-compiling, we need to build the protobuf libraries for the
target architecture, yet we need to use a protoc executable built for
the host architecture.

cmake/protobuf.cmake

@@ -39,7 +39,11 @@ if(gRPC_PROTOBUF_PROVIDER STREQUAL "module")
     endif()
     if(TARGET protoc)
       set(_gRPC_PROTOBUF_PROTOC protoc)
-      set(_gRPC_PROTOBUF_PROTOC_EXECUTABLE $<TARGET_FILE:protoc>)
+      if(CMAKE_CROSSCOMPILING)
+        find_program(_gRPC_PROTOBUF_PROTOC_EXECUTABLE protoc)
+      else()
+        set(_gRPC_PROTOBUF_PROTOC_EXECUTABLE $<TARGET_FILE:protoc>)
+      endif()
     endif()
     # For well-known .proto files distributed with protobuf
     set(_gRPC_PROTOBUF_WELLKNOWN_INCLUDE_DIR "${PROTOBUF_ROOT_DIR}/src")
@@ -73,10 +77,18 @@ elseif(gRPC_PROTOBUF_PROVIDER STREQUAL "package")
     endif()
     if(TARGET protobuf::protoc)
       set(_gRPC_PROTOBUF_PROTOC protobuf::protoc)
-      set(_gRPC_PROTOBUF_PROTOC_EXECUTABLE $<TARGET_FILE:protobuf::protoc>)
+      if(CMAKE_CROSSCOMPILING)
+        find_program(_gRPC_PROTOBUF_PROTOC_EXECUTABLE protoc)
+      else()
+        set(_gRPC_PROTOBUF_PROTOC_EXECUTABLE $<TARGET_FILE:protobuf::protoc>)
+      endif()
     else()
       set(_gRPC_PROTOBUF_PROTOC ${PROTOBUF_PROTOC_EXECUTABLE})
-      set(_gRPC_PROTOBUF_PROTOC_EXECUTABLE ${PROTOBUF_PROTOC_EXECUTABLE})
+      if(CMAKE_CROSSCOMPILING)
+        find_program(_gRPC_PROTOBUF_PROTOC_EXECUTABLE protoc)
+      else()
+        set(_gRPC_PROTOBUF_PROTOC_EXECUTABLE ${PROTOBUF_PROTOC_EXECUTABLE})
+      endif()
     endif()
     set(_gRPC_FIND_PROTOBUF "if(NOT Protobuf_FOUND AND NOT PROTOBUF_FOUND)\n  find_package(Protobuf ${gRPC_PROTOBUF_PACKAGE_TYPE})\nendif()")
   endif()