Use version check instead of bumping minimum cmake version

Author: TonyWelte

rosidl_generator_type_description/cmake/rosidl_generator_type_description_generate_interfaces.cmake

@@ -24,7 +24,7 @@
 # the search will prefer /usr/bin/python3 over /usr/bin/python3.11.  And that
 # latter functionality is only available in CMake 3.20 or later, so we need
 # at least that version.
-cmake_minimum_required(VERSION 3.27) # Required by option DEPENDS_EXPLICIT_ONLY of add_custom_command
+cmake_minimum_required(VERSION 3.20)
 cmake_policy(SET CMP0094 NEW)
 set(Python3_FIND_UNVERSIONED_NAMES FIRST)
 
@@ -78,6 +78,12 @@ rosidl_write_generator_arguments(
   INCLUDE_PATHS "${_dependency_paths}"
 )
 
+if(${CMAKE_VERSION} VERSION_GREATER_EQUAL 3.27)
+  set(_dep_explicit_only DEPENDS_EXPLICIT_ONLY)
+else()
+  set(_dep_explicit_only "")
+endif()
+
 # Create custom command and target to generate the hash output
 add_custom_command(
   COMMAND Python3::Interpreter
@@ -88,7 +94,7 @@ add_custom_command(
   DEPENDS ${target_dependencies}
   COMMENT "Generating type hashes for ROS interfaces"
   VERBATIM
-  DEPENDS_EXPLICIT_ONLY
+  ${_dep_explicit_only}
 )
 
 set(_target "${rosidl_generate_interfaces_TARGET}__rosidl_generator_type_description")

rosidl_typesupport_introspection_c/cmake/rosidl_typesupport_introspection_c_generate_interfaces.cmake

@@ -88,12 +88,18 @@ rosidl_write_generator_arguments(
 # the search will prefer /usr/bin/python3 over /usr/bin/python3.11.  And that
 # latter functionality is only available in CMake 3.20 or later, so we need
 # at least that version.
-cmake_minimum_required(VERSION 3.27) # Required by option DEPENDS_EXPLICIT_ONLY of add_custom_command
+cmake_minimum_required(VERSION 3.20)
 cmake_policy(SET CMP0094 NEW)
 set(Python3_FIND_UNVERSIONED_NAMES FIRST)
 
 find_package(Python3 REQUIRED COMPONENTS Interpreter)
 
+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_header_files} ${_generated_source_files}
   COMMAND Python3::Interpreter
@@ -102,7 +108,7 @@ add_custom_command(
   DEPENDS ${target_dependencies}
   COMMENT "Generating C introspection for ROS interfaces"
   VERBATIM
-  DEPENDS_EXPLICIT_ONLY
+  ${_dep_explicit_only}
 )
 
 # generate header to switch between export and import for a specific package

rosidl_typesupport_introspection_cpp/cmake/rosidl_typesupport_introspection_cpp_generate_interfaces.cmake

@@ -88,12 +88,18 @@ rosidl_write_generator_arguments(
 # the search will prefer /usr/bin/python3 over /usr/bin/python3.11.  And that
 # latter functionality is only available in CMake 3.20 or later, so we need
 # at least that version.
-cmake_minimum_required(VERSION 3.27) # Required by option DEPENDS_EXPLICIT_ONLY of add_custom_command
+cmake_minimum_required(VERSION 3.20)
 cmake_policy(SET CMP0094 NEW)
 set(Python3_FIND_UNVERSIONED_NAMES FIRST)
 
 find_package(Python3 REQUIRED COMPONENTS Interpreter)
 
+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_header_files} ${_generated_source_files}
   COMMAND Python3::Interpreter
@@ -102,7 +108,7 @@ add_custom_command(
   DEPENDS ${target_dependencies}
   COMMENT "Generating C++ introspection for ROS interfaces"
   VERBATIM
-  DEPENDS_EXPLICIT_ONLY
+  ${_dep_explicit_only}
 )
 
 set(_target_suffix "__rosidl_typesupport_introspection_cpp")