devel/poco: Modernize port Makefile

• Replace CMAKE_ARGS and <opt>_CMAKE_[ON|OFF] by CMAKE_[ON|OFF] and <opt>_CMAKE_BOOL[_OFF] for handling cmake BOOL knobs
• Drop a CONFLICTS to a port that was removed in 2019
• As advised by portclippy, put all options descriptions in their own block

Approved by: portmgr (blanket)