Add SRCURI configuration for manual

* configure.ac (SET_SRCURI): Provide source root to manual.
  (--with-srcuri): Add configure option.
* doc/config.texi.in (SET_SRCURI): Add configuration value.
* doc/liza.texi: Add warning if SET_SRCURI is unset when DEVNOTES is set.

configure.ac

@@ -50,6 +50,16 @@ AC_ARG_ENABLE(
 
 AC_SUBST([SET_DEVNOTES], [$set_devnotes])
 
+set_srcuri='@c @set SRCURI <source code uri>'
+AC_ARG_WITH(
+  [srcuri],
+  [AS_HELP_STRING([--with-srcuri],
+                  [root URI for Liza source code cross-references
+                   (disabled by default)])],
+  [test "x$withval" = x || set_srcuri="@set SRCURI $withval"])
+
+AC_SUBST([SET_SRCURI], [$set_srcuri])
+
 # generate files from their *.in counterparts
 AC_CONFIG_FILES([Makefile package.json
                  doc/Makefile doc/config.texi

doc/config.texi.in

@@ -2,3 +2,6 @@
 
 @c whether to include notes for Liza developers
 @SET_DEVNOTES@
+
+@c URI to root of source tree for manual cross-references
+@SET_SRCURI@

doc/liza.texi

@@ -58,6 +58,15 @@ Free Documentation License''.
       to @command{configure}.}
   For an index of notes@comma{} see @ref{Developer Notes Index}.}
 
+@ifclear SRCURI
+@devnotice{
+  Location of Liza source code is unknown.
+  Source cross-references have been disabled.@footnote{
+      To enable source code cross-references, provide the URI to the
+        root (as you want it to be seen in the manual) via
+        @option{--with-srcuri=<uri>} to @command{configure}.}}
+@end ifclear
+
 @contents
 
 @ifnottex