Use FindGengetopt.cmake provided by RTK

[LucasGandel]

• Update RTK source code to add the RTK_SOURCE_DIR variable in RTK_EXPORT_CODE_BUILD
• Append ${RTK_SOURCE_DIR}/Cmake to the CMAKE_MODULE_PATH in PCT and remove custom FindGengetopt.cmake

[SimonRit]

@LucasGandel Have you ever progressed on this issue and, if not, is it in your hands or in our hands now?