manual: accept absence of the asciidoctor-pdf exectuable

manual: accept absence of the asciidoctor-pdf exectuable

The asciidoctor(1) executable is considered mandatory when building the
Pulseview manual. The asciidoctor-pdf(1) executable is not universally
available (is missing in Debian), accept its absence, avoid execution
failure in that case.

This implementation replaces the actual .txt to .pdf conversion with a
mere echo(1) message, which may go unnoticed in verbose build output.
"make --no-print-directories manual" may be required to remain aware.