make: Use asciidoctor by default

The final release of asciidoc was on Sep 29, 2017 and the development
is continued under asciidoctor.

With commit 0493724 (Documentation: Allow to use asciidoctor for
formatting man pages) was added support for this tool by introducing
USE_ASCIIDOCTOR.

However, using asciidoctor by default might be a better option. With
this change CRIU will use asciidoctor if installed. Otherwise, it will
fallback to asciidoc.

Signed-off-by: Radostin Stoyanov <rstoyanov1@gmail.com>

scripts/build/Dockerfile.alpine

@@ -36,7 +36,7 @@ RUN apk add \
 	bash \
 	go \
 	e2fsprogs \
-	asciidoc xmlto
+	asciidoctor
 
 # The rpc test cases are running as user #1000, let's add the user
 RUN adduser -u 1000 -D test

scripts/build/Dockerfile.centos

@@ -32,7 +32,7 @@ RUN yum install -y \
 	which \
 	e2fsprogs \
 	python2-pip \
-	asciidoc xmlto
+	rubygem-asciidoctor
 
 COPY . /criu
 WORKDIR /criu

scripts/build/Dockerfile.fedora.tmpl

@@ -33,7 +33,7 @@ RUN dnf install -y \
 	tar \
 	which \
 	e2fsprogs \
-	asciidoc xmlto \
+	rubygem-asciidoctor \
 	kmod
 
 # Replace coreutils-single with "traditional" coreutils

scripts/nmk/scripts/tools.mk

@@ -35,6 +35,8 @@ CTAGS		:= ctags
 export RM HOSTLD LD HOSTCC CC CPP AS AR STRIP OBJCOPY OBJDUMP
 export NM SH MAKE MKDIR AWK PERL PYTHON SH CSCOPE
 
+export USE_ASCIIDOCTOR ?= $(shell which asciidoctor 2>/dev/null)
+
 #
 # Footer.
 ____nmk_defined__tools = y

scripts/travis/travis-tests

@@ -4,7 +4,7 @@ set -x -e
 TRAVIS_PKGS="protobuf-c-compiler libprotobuf-c0-dev libaio-dev
 		libprotobuf-dev protobuf-compiler libcap-dev
 		libnl-3-dev gcc-multilib gdb bash python-protobuf
-		libnet-dev util-linux asciidoc xmlto libnl-route-3-dev"
+		libnet-dev util-linux asciidoctor libnl-route-3-dev"
 
 travis_prep () {
 	[ -n "$SKIP_TRAVIS_PREP" ] && return