Clarify that graphviz is normally optional

This is a hard dep iff the svg files need to be updated.

configure.ac

@@ -60,7 +60,7 @@ AC_ARG_WITH([dot],
   [AC_CHECK_PROGS(DOT, [dot], [no])
    if test "x$DOT" = "xno"; then
      AC_MSG_WARN(
-       [dot not found - cannot generate graphviz output for documentation.])
+       [dot not found - build will fail if svg files are out of date.])
    fi])
 AM_CONDITIONAL(HAVE_DOT,[test "x$DOT" != "xno"])
 

doc/sphinx/installation/install.rst

@@ -88,6 +88,9 @@ installed. On a Debian or Ubuntu system these are:
 * `pkg-config`
 * `python-docutils`
 * `python-sphinx`
+
+Optionally, to rebuild the svg files:
+
 * `graphviz`
 
 Build dependencies on Red Hat / CentOS
@@ -108,6 +111,9 @@ packages installed:
 * `pkgconfig`
 * `python-docutils`
 * `python-sphinx`
+
+Optionally, to rebuild the svg files:
+
 * `graphviz`
 
 Build dependencies on a SmartOS Zone
@@ -116,8 +122,11 @@ Build dependencies on a SmartOS Zone
 As of SmartOS pkgsrc 2015Q4, install the following packages::
 
 	pkgin in autoconf automake libedit libtool ncurses \
-		 pcre graphviz py27-sphinx python27 gmake gcc49 \
-		 pkg-config
+		 pcre py27-sphinx python27 gmake gcc49 pkg-config
+
+Optionally, to rebuild the svg files:
+
+        pkgin in graphviz
 
 Optionally, to pull from a repository::