Add build rule for build-octave (bug #49503). * src/build-octave.mk: Add new build rule for build-octave which can be used for cross-building binary packages that depend on Octave as a build tool. * dist-files.mk: Add new file to list. * index.html: Add new package to list. * src/of-communications.mk, src/of-image.mk, src/of-mapping.mk, src/of-optiminterp.mk, src/of-sparsersb.mk, src/of-statistics.mk, src/of-windows.mk: Add optional dependency on build-octave. * configure.ac: Add new configure switch "--disable-system-octave" that is needed to build the build-octave package. * Makefile.in: Add variable "USE_SYSTEM_OCTAVE". Exclude build-octave from the default build tools and build it only if a package explicitly depends on it. Display warning about missing native Octave version only if necessary.

--- a/index.html	Tue Sep 08 16:04:48 2020 +0200
+++ b/index.html	Tue Sep 08 23:04:38 2020 +0200
@@ -1050,6 +1050,10 @@
         <td class="website"><a href="https://ninja-build.org/">build-ninja</a></td>
     </tr>
     <tr>
+        <td class="package">build-octave</td>
+        <td class="website">Minimal version of <a href="http://www.octave.org">Octave</a> for cross-compiling some binary packages</td>
+    </tr>
+    <tr>
         <td class="package">build-perl</td>
         <td class="website"><a href="http://www.perl.org">perl</a></td>
     </tr>