Merge 'build: relax the version constraint for Protobuf' from Kefu Chai

as higher versions of Protobuf does not claim to be compatible with
v2.5.0, so we should not specify a version number which is acceptable
by Seastar as a parameter of `find_package()`. and instead, we just
check the version number after calling `find_package()` in config mode.
if we find the package using config mode, we search it again, for
printing out the version number to the stdout, otherwise we search
using its C++ API version.

Refs ##2113

Closes #2132

* github.com:scylladb/seastar:
  build: relax the version constraint for Protobuf
  build: use protobuf::protoc for the path to protoc compiler

CMakeLists.txt

@@ -417,10 +417,11 @@ function (seastar_generate_protobuf)
  set (source_out ${args_OUT_DIR}/${in_file_name}.pb.cc)
 
  add_custom_command (
- DEPENDS ${args_IN_FILE}
+ DEPENDS ${args_IN_FILE} protobuf::protoc
  OUTPUT ${header_out} ${source_out}
  COMMAND ${CMAKE_COMMAND} -E make_directory ${args_OUT_DIR}
- COMMAND ${Protobuf_PROTOC_EXECUTABLE} --cpp_out=${args_OUT_DIR} -I${in_file_dir} ${args_IN_FILE})
+ COMMAND protobuf::protoc
+ ARGS --cpp_out=${args_OUT_DIR} -I${in_file_dir} ${args_IN_FILE})
 
  add_custom_target (${args_TARGET}
  DEPENDS

cmake/SeastarDependencies.cmake

@@ -159,14 +159,11 @@ macro (seastar_find_dependencies)
  # ProtobufConfig.cmake provided by protobuf defines this linkage. so we try
  # the CMake package configuration file first, and fall back to CMake's
  # FindProtobuf module.
- find_package (Protobuf 2.5.0
- QUIET CONFIG)
- if (Protobuf_FOUND)
+ find_package (Protobuf QUIET CONFIG)
+ if (Protobuf_FOUND AND Protobuf_VERSION VERSION_GREATER_EQUAL 2.5.0)
  # do it again, so the message is printed when the package is found
- find_package(Protobuf 2.5.0
- CONFIG REQUIRED)
+ find_package(Protobuf CONFIG REQUIRED)
  else ()
- find_package(Protobuf 2.5.0
- REQUIRED)
+ find_package(Protobuf 2.5.0 REQUIRED)
  endif ()
 endmacro ()