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.

1 files changed, 4 insertions, 1 deletions

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>