Add awk one-liner to check helpdb.jl build

`make -C doc helpdb.jl` now runs a short awk script that counts the
number of consecutive empty lines in helpdb.jl. If two or more
consecutive empty lines are detected, the awk script throws an error and
prevents `make` from overwriting the current helpdb.jl.

The purpose of this PR is to avoid accidentally check in builds of helpdb.jl with extraneous newlines which occur with certain versions of Sphinx.