Use asciidoctor to build the manpages

[cbiedl]

Like in latchset/tang#32 - asciidoc is EOL, see for example https://github.com/asciidoc/asciidoc/releases.

Switching to asciidoctor is fairly simple:

--- a/meson.build
+++ b/meson.build
@@ -61,10 +61,10 @@
   warning('Will not run tests due to missing dependencies!')
 endif
 
-a2x = find_program('a2x', required: false)
-if a2x.found()
+asciidoctor = find_program('asciidoctor', required: false)
+if asciidoctor.found()
   custom_target('nagios-tang.1',
-    command: [a2x, '-f', 'manpage', '-D', meson.current_build_dir(), '@INPUT@'],
+    command: [asciidoctor, '--attribute', 'reproducible', '--backend=manpage', '-D', meson.current_build_dir(
), '@INPUT@'],
     install_dir: join_paths(get_option('mandir'), 'man1'),
     input: 'nagios-tang.1.adoc',
     output: 'nagios-tang.1',

[cbiedl]

Strike that, asciidoc is still alive.