configure.ac: improve database dependency checks

Abort if --enable-libmpdclient or --enable-upnp are used with
--disable-database, instead of ignoring the mismatch silently.
configure.ac: auto-disable plugins that require GLib when --disable-glib is used
fs/Charset: work around clang's -Wunused-const-variable

MPD_PATH_MAX_UTF8 is only used by GLib-specific code currently.
test/test_archive: don't use GLib
configure.ac: require xmlto for --enable-documentation
configure.ac: show DSD in result