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 quaternion.orig/src/Makefile quaternion/src/Makefile
--- quaternion.orig/src/Makefile	2015-05-10 07:14:09.000000000 -0400
+++ quaternion/src/Makefile	2015-05-10 07:15:03.000000000 -0400
@@ -1,11 +1,13 @@
+MKOCTFILE ?= mkoctfile
+
 all: is_real_array.oct
 
 # helper
 is_real_array.oct: is_real_array.cc
-	mkoctfile is_real_array.cc
+	$(MKOCTFILE) is_real_array.cc
 
 clean:
 	rm -rf *.o core octave-core *.oct *~
 
 realclean: clean
-	rm -rf *.a
\ No newline at end of file
+	rm -rf *.a