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/dist-files.mk	Tue Sep 08 16:04:48 2020 +0200
+++ b/dist-files.mk	Tue Sep 08 23:04:38 2020 +0200
@@ -41,6 +41,7 @@
   build-meson.mk \
   build-msvctools.mk \
   build-ninja.mk \
+  build-octave.mk \
   build-perl.mk \
   build-pkg-config-1-fixes.patch \
   build-pkg-config.mk \