Support separate lib and include install directories for libzmq.

Added --with-libzmq-include-dir and --with-libzmq-lib-dir configure options.
If --with-libzmq is set, both CFLAGS and LDFLAGS are set using that prefix.
If --with-libzmq-include-dir or --with-libzmq-lib-dir are set, the CFLAGS
and LDFLAGS are set using those prefixes, respectively.  The new configure
options override --with-libzmq, if set.