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.

diff -ur statistics-1.4.0.orig/utils/conditional_installation.m statistics-1.4.0/utils/conditional_installation.m
--- statistics-1.4.0.orig/utils/conditional_installation.m	2018-05-21 07:55:58.881279480 -0400
+++ statistics-1.4.0/utils/conditional_installation.m	2018-05-21 07:58:13.197213866 -0400
@@ -24,7 +24,7 @@
   ## build system, and this file here has to be configured as follows:
   ##
   ## Set this to true if you have considered the next step.
-  configured = false;
+  configured = true;
   ##
   ## If core Octave called at the build system has the same statistics
   ## functions as the core Octave you build for, leave the following