Make execline dependency optional

 This includes:
 - adding a configure option to disable execline support
 - duplicating el_semicolon() and el_getstrict() into the s6 library
when execline is disabled at build time, making a compat shim and
using it where needed (s6-svlisten, s6-ftrig-listen)
 - Creating alternatives for execlineb script spawning:
   * s6-log: '?' directive (done in the previous commit)
   * s6-notifyoncheck: require hardcoding the check in ./data/check
   * s6-ipcserver-access: no support for exec files, add a warning
 - Updating the relevant parts of the doc

 --enable-execline will remain the default, and whiners can choke
on their tears.

4 files changed, 17 insertions, 6 deletions

diff --git a/doc/index.html b/doc/index.html
index 060f0ed..341a936 100644
--- a/doc/index.html
+++ b/doc/index.html
@@ -86,8 +86,11 @@ with s6</a> </li>
 2.9.1.0 or later. It's a build-time requirement. It's also a run-time
 requirement if you link against the shared version of the skalibs
 library. </li>
- <li> <a href="//skarnet.org/software/execline/">execline</a> version
-2.5.3.0 or later. It's a build-time and run-time requirement. </li>
+ <li> Optional: <a href="//skarnet.org/software/execline/">execline</a> version
+2.5.3.0 or later. When s6 is built with execline support (which is the default,
+execline is a build-time requirement, and also a run-time requirement for
+certain binaries that spawn scripts interpreted with
+<a href="//skarnet.org/software/execline/execlineb.html">execlineb</a>. </li>
 </ul>
 
 <h3> Licensing </h3>
diff --git a/doc/s6-ipcserver-access.html b/doc/s6-ipcserver-access.html
index b09d74a..2d35d84 100644
--- a/doc/s6-ipcserver-access.html
+++ b/doc/s6-ipcserver-access.html
@@ -168,7 +168,10 @@ is modified according to the contents of <tt>env</tt>. </li>
  <li> An <tt>exec</tt> file containing <em>newprog</em> completely
 bypasses the rest of s6-ipcserver-access' command line. After
 environment modifications, if any, s6-ipcserver-access execs into
-<tt><a href="//skarnet.org/software/execline/execlineb.html">execlineb</a> -c <em>newprog</em></tt>. </li>
+<tt><a href="//skarnet.org/software/execline/execlineb.html">execlineb</a> -c <em>newprog</em></tt>.
+Please be aware that the <tt>exec</tt> file functionality is only supported
+when s6 has been built with execline support. Otherwise, a warning message
+is printed and executable diversion is <em>not</em> performed. </li>
 </ul>
 
 </body>
diff --git a/doc/s6-log.html b/doc/s6-log.html
index 5cc4529..f042c70 100644
--- a/doc/s6-log.html
+++ b/doc/s6-log.html
@@ -234,14 +234,16 @@ padded with newlines so it is always <em>statussize</em> bytes long. 0 means
 an unpadded, unlimited status file. By default, <em>statussize</em> is 1001. </li>
  <li> <strong>!<em>processor</em></strong>: registers
 <tt>execlineb -Pc <em>processor</em></tt> as a processor for the next logdirs;
-<tt>execlineb</tt> must be found in s6-log's PATH.
+<tt>execlineb</tt> must be found in s6-log's PATH. This directive is only
+supported if s6 has been built with execline support; otherwise, it yields a
+syntax error at starting time.
 If <em>processor</em> is empty, no processor will be set for the next logdirs.
 By default, no processor is set. </li>
  <li> <strong>?<em>processor</em></strong>: registers
 <tt>/bin/sh -c <em>processor</em></tt> as a processor for the next logdirs.
 It is just like the <tt>!</tt> directive, except that the processor string
 is interpreted by <tt>/bin/sh</tt>, not <tt>execlineb</tt>. It is useful
-for people who have built s6 without execline support. </li>
+for people who want to build s6 without execline support. </li>
  <li> <strong>t</strong>: the logged line will be prepended with a
 <a href="//skarnet.org/software/skalibs/libstddjb/tai.html">TAI64N
 timestamp</a> (and a space) before being processed by the next action
diff --git a/doc/s6-notifyoncheck.html b/doc/s6-notifyoncheck.html
index 2b6bfc6..e154da0 100644
--- a/doc/s6-notifyoncheck.html
+++ b/doc/s6-notifyoncheck.html
@@ -114,7 +114,10 @@ instead of <tt>./data/check</tt>. The <em>checkprog</em> string will be parsed b
 can contain a full command line. This option is mainly useful is the program
 used to poll the service is very simple and can be inlined as a simple
 command line, to avoid needing to manage a whole script and a <tt>./data/check</tt>
-file. </li>
+file. This option is only supported if the
+<a href="//skarnet.org/software/execline/">execline</a> package is installed;
+if it is not, the <tt>-c</tt> option cannot be used and the checking
+script must always be placed in <tt>./data/check</tt>. </li>
 </ul>
 
 <h2> Usage </h2>