Add DEPENDS_EXPLICIT_ONLY to remove implicit dependencies (#238)

* Add DEPENDS_EXPLICIT_ONLY to remove implicit dependencies

Signed-off-by: Anthony Welte <tony.welte@gmail.com>

* Use version check instead of bumping minimum cmake version

Signed-off-by: Anthony Welte <tony.welte@gmail.com>

---------

Signed-off-by: Anthony Welte <tony.welte@gmail.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: TonyWelte

rosidl_generator_py/cmake/custom_command.cmake

@@ -18,6 +18,12 @@
 # CMake does not allow `add_custom_command()` to depend on files generated in
 # a different CMake subdirectory, and this command is invoked after an
 # add_subdirectory() call.
+if(${CMAKE_VERSION} VERSION_GREATER_EQUAL 3.27)
+  set(_dep_explicit_only DEPENDS_EXPLICIT_ONLY)
+else()
+  set(_dep_explicit_only "")
+endif()
+
 add_custom_command(
   OUTPUT ${_generated_extension_files} ${_generated_py_files} ${_generated_c_files}
   # This assumes that python_cmake_module was found, which is always the case since this is only
@@ -28,6 +34,7 @@ add_custom_command(
   DEPENDS ${target_dependencies} ${rosidl_generate_interfaces_TARGET}
   COMMENT "Generating Python code for ROS interfaces"
   VERBATIM
+  ${_dep_explicit_only}
 )
 
 if(TARGET ${rosidl_generate_interfaces_TARGET}${_target_suffix})