build: properly test for libplumb, remove useless -lplumbgpl

Add an AC_CHECK_LIB test for cl_log, autoconf automatically adds
this to LIBS. No need to explicitly specifiy -lplumbgpl.

Fixes Debian build warning:
dpkg-shlibdeps: warning: dependency on libplumbgpl.so.2 could be avoided if "debian/booth/usr/sbin/boothd" were not uselessly linked against it (they use none of its symbols).