Actually, as the User Guide explains, it is not sufficient to relink (or use dynamic linking), since the applications also need to be compiled with the changed values for MAXDEPTH/MAXKEYLEN - otherwise they would be using a different definition of confd_hkeypath_t than the library is using, and e.g. data provider callbacks receiving a hkeypath built by the library as a parameter would not be able to interpret it correctly. Such a mismatch between applcation and library will actually be detected by the confd_init() call, which will terminate the application with an error message.
As for “automatic”, Makefiles in e.g. the examples could of course have more dependencies, but depending on how the change is done - the User Guide discusses some alternatives - this may not be sufficient. E.g. a dependency on $CONFD_DIR/include/confd_lib.h will only help if that file is actually modified, which may be undesirable.