cmake: Add cross-toolchain support

- Add host build of protoc

cmake/host.cmake

@@ -0,0 +1,56 @@
+if(NOT CMAKE_CROSSCOMPILING)
+  set(PROTOC_PRG protobuf::protoc)
+  return()
+endif()
+
+message(STATUS "Subproject: HostTools...")
+
+#configure_file(
+#	${CMAKE_CURRENT_SOURCE_DIR}/host.CMakeLists.txt
+#  ${CMAKE_CURRENT_BINARY_DIR}/host_tools/CMakeLists.txt)
+#
+#execute_process(
+#  COMMAND ${CMAKE_COMMAND} -S. -Bbuild -DCMAKE_BUILD_TYPE=Release -G "${CMAKE_GENERATOR}" .
+#  RESULT_VARIABLE result
+#  WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/host_tools)
+#if(result)
+#  message(FATAL_ERROR "CMake step for host tools failed: ${result}")
+#endif()
+#execute_process(
+#  COMMAND ${CMAKE_COMMAND} --build .
+#  RESULT_VARIABLE result
+#  WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/host_tools)
+#if(result)
+#  message(FATAL_ERROR "Build step for host tools failed: ${result}")
+#endif()
+
+#file(COPY
+#	${CMAKE_CURRENT_SOURCE_DIR}/cmake/host.CMakeLists.txt
+#  DESTINATION ${CMAKE_CURRENT_BINARY_DIR}/host_tools)
+
+configure_file(
+	${CMAKE_CURRENT_SOURCE_DIR}/cmake/host.CMakeLists.txt
+  ${CMAKE_CURRENT_BINARY_DIR}/host_tools/CMakeLists.txt
+  COPYONLY)
+
+add_custom_command(
+  OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/host_tools
+  #COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/host.CMakeLists.txt CMakeLists.txt
+  COMMAND ${CMAKE_COMMAND} -E remove_directory build
+  COMMAND ${CMAKE_COMMAND} -S. -Bbuild -DCMAKE_BUILD_TYPE=Release -DCMAKE_RUNTIME_OUTPUT_DIRECTORY:PATH=${CMAKE_CURRENT_BINARY_DIR}/host_tools/bin
+  COMMAND ${CMAKE_COMMAND} --build build --config Release -v
+  WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/host_tools
+)
+
+add_custom_target(host_tools
+  DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/host_tools
+  WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
+
+add_executable(host_protoc IMPORTED GLOBAL)
+set_target_properties(host_protoc PROPERTIES
+  IMPORTED_LOCATION ${CMAKE_CURRENT_BINARY_DIR}/host_tools/bin/protoc)
+
+add_dependencies(host_protoc host_tools)
+set(PROTOC_PRG host_protoc)
+
+message(STATUS "Subproject: HostTools...DONE")