Documentation: move dblatex arguments into variable

Documentation: move dblatex arguments into variable

Our dblatex invocation uses several style components from the AsciiDoc
distribution, but those components are not available when building with
Asciidoctor.  Move the command line arguments into a variable so it can
be overridden by the user or makefile configuration options.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>