CMakeLists: add_custom_target(update-api), aux_source_directory to load files list
CMakeLists: add_custom_target(update-api), aux_source_directory to load files list

- cmake --target update-api can be used to update the api files
- aux_source_directory() is used to enumerate generated files instead
  of apifiles.txt (we wouldn't be able to rerun CMake on the changed
  files list anyway).

QMatrixClient/libqmatrixclient,Fxrh/libqmatrixclient,QMatrixClient/libqmatrixclient