Build: add support for AsciiDoc's successor, Asciidoctor

Everything seems to be built just fine when Asciidoctor is present in
the build environment and AsciiDoc is not (as it takes the precedence),
except for Pacemake Explained that needs some more tweaking to be
usable with Asciidoctor:

Ch-Options.xml:125: validity error : Element emphasis is not declared in literal list of possible children
Ch-Options.xml:419: validity error : Element xref is not declared in literal list of possible children
Ch-Options.xml:716: validity error : Element xref is not declared in literal list of possible children
Ch-Options.xml:824: validity error : Element emphasis is not declared in literal list of possible children
Ch-Options.xml:860: validity error : Element emphasis is not declared in literal list of possible children
Ch-Options.xml:896: validity error : Element emphasis is not declared in literal list of possible children
Ch-Resources.xml:608: validity error : Element xref is not declared in literal list of possible children
Ch-Constraints.xml:525: validity error : Element xref is not declared in literal list of possible children
Ch-Advanced-Options.xml:662: validity error : Element xref is not declared in literal list of possible children
Ch-Stonith.xml:153: validity error : Element emphasis is not declared in literal list of possible children
Ch-Stonith.xml:153: validity error : Element emphasis is not declared in literal list of possible children

Also remove superfluous "inverse grep" conditionalizing amounting to
a thinko how that's supposed to work. Another logical problem was with
"it's OK to generate the final product of the makefile's target early
and adjust it in-place in the steps of the recipe to follow" broken
assumption.