Set hints to find the python version we actually want. (#287)

The comment in the commit explains the reasoning behind it.

Signed-off-by: Chris Lalancette <[email protected]>

qt_gui_cpp/src/qt_gui_cpp_sip/CMakeLists.txt

@@ -35,6 +35,22 @@ ament_get_recursive_properties(deps_include_dirs deps_libraries ${pluginlib_TARG
 list(APPEND deps_include_dirs ${TinyXML2_INCLUDE_DIRS})
 list(APPEND deps_libraries ${TinyXML2_LIBRARIES})
 
+# By default, without the settings below, find_package(Python3) will attempt
+# to find the newest python version it can, and additionally will find the
+# most specific version. For instance, on a system that has
+# /usr/bin/python3.10, /usr/bin/python3.11, and /usr/bin/python3, it will find
+# /usr/bin/python3.11, even if /usr/bin/python3 points to /usr/bin/python3.10.
+# The behavior we want is to prefer the "system" installed version unless the
+# user specifically tells us othewise through the Python3_EXECUTABLE hint.
+# Setting CMP0094 to NEW means that the search will stop after the first
+# python version is found. Setting Python3_FIND_UNVERSIONED_NAMES means that
+# 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 3.20)
+cmake_policy(SET CMP0094 NEW)
+set(Python3_FIND_UNVERSIONED_NAMES FIRST)
+
 find_package(Python3 REQUIRED COMPONENTS Development)
 
 set(_qt_gui_cpp_sip_LIBRARIES