CMake: overhaul and improve scripts

- Bump minimum required version and
make use of more modern language features
- Rely more on target_...() commands to establish dependency
relationships between targets rather than directory property commands
- Improve libtorrent package discovery
- Enable and handle application features more explicitly
- Improve user-facing output
- Fix various compilation issues on Windows (MSVC and MinGW) and macOS
- Improve handling of translations
- Add explanatory comments where relevant
- Make CMake scripts fully independent of qmake files/details
- Remove old functions/macros

cmake/Modules/FindSystemd.cmake

@@ -8,7 +8,7 @@ find_package(PkgConfig QUIET REQUIRED)
 
 if (NOT SYSTEMD_FOUND)
     pkg_check_modules(SYSTEMD "systemd")
-endif(NOT SYSTEMD_FOUND)
+endif()
 
 if (SYSTEMD_FOUND AND "${SYSTEMD_SERVICES_INSTALL_DIR}" STREQUAL "")
     execute_process(COMMAND ${PKG_CONFIG_EXECUTABLE}
@@ -23,4 +23,4 @@ endif()
 
 if (SYSTEMD_FOUND)
     message(STATUS "systemd services install dir: ${SYSTEMD_SERVICES_INSTALL_DIR}")
-endif(SYSTEMD_FOUND)
+endif()