Build: protoc in cross compile case

thirdparty/build-protobuf.cmake

@@ -3,6 +3,8 @@ set(protobuf_MSVC_STATIC_RUNTIME OFF CACHE BOOL "My option" FORCE)
 if(UNIX)
   set(protobuf_BUILD_SHARED_LIBS ON CACHE BOOL "My option" FORCE)
 endif()
+set(Protobuf_PROTOC_EXECUTABLE "" CACHE FILEPATH "Path to protoc executable")
+
 add_subdirectory(thirdparty/protobuf/cmake)
 
 if (NOT TARGET protobuf::libprotobuf)
@@ -18,20 +20,22 @@ if (NOT is_imported)
   )
 endif()
 
-if (NOT TARGET protoc AND CMAKE_CROSSCOMPILING)
+if (CMAKE_CROSSCOMPILING)
   # In cross compile scenario, we don't build protoc, so we need
   # to find and import it so downstream usage will use the host compiler.
 
   # find_program will search the host paths if CMAKE_FIND_ROOT_PATH_MODE_PROGRAM 
   # is configured correctly (see toolchain file)
-  find_program(protoc_path NAMES protoc REQUIRED)
-  add_executable(protoc IMPORTED)
-  set_target_properties(protoc PROPERTIES IMPORTED_LOCATION ${protoc_path})
-  set_target_properties(protoc PROPERTIES IMPORTED_GLOBAL TRUE)
+  if (NOT Protobuf_PROTOC_EXECUTABLE)
+    find_program(Protobuf_PROTOC_EXECUTABLE NAMES protoc_host REQUIRED)
+  endif ()
+  add_executable(protoc_host IMPORTED)
+  set_target_properties(protoc_host PROPERTIES IMPORTED_LOCATION ${Protobuf_PROTOC_EXECUTABLE})
+  set_target_properties(protoc_host PROPERTIES IMPORTED_GLOBAL TRUE)
 
-  add_executable(protobuf::protoc ALIAS protoc)
+  add_executable(protobuf::protoc ALIAS protoc_host)
 
-  message(STATUS "protoc has been automatically imported from ${protoc_path} for crosscompiling.")
+  message(STATUS "protoc has been set to ${Protobuf_PROTOC_EXECUTABLE} for crosscompiling.")
 
 elseif(NOT TARGET protobuf::protoc)
   add_executable(protobuf::protoc ALIAS protoc)
@@ -46,5 +50,4 @@ if (NOT is_imported)
   )
 endif()
 
-set(Protobuf_PROTOC_EXECUTABLE protoc)
-set(Protobuf_VERSION 3.11.4)
+set(Protobuf_VERSION 3.11.4)