make: allow the asciidoc command to be altered

Prefering the reference AsciiDoc implementation for now, but as the
Python 2.7 EOL nears, at least have an easy out if the software can’t
be used anymore.

Author: chungy

Makefile

@@ -1,6 +1,7 @@
 # Makefile for reflac
 
-# asciidoc is required to generate the manpage.
+# This can be changed to "asciidoctor -b manpage" if necessary.
+ASCIIDOC?=a2x -f manpage
 
 # Autotools-like prefix, mandir, and DESTDIR variables can be
 # overriden to control where files are installed.
@@ -13,7 +14,7 @@ all: man
 man: reflac.1
 
 reflac.1: reflac.adoc
-	a2x -f manpage reflac.adoc
+	$(ASCIIDOC) reflac.adoc
 
 install: reflac.1
 	install -Dm 755 reflac "$(target)/bin/reflac"