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 fits-1.0.7.orig/src/Makefile fits-1.0.7/src/Makefile
--- fits-1.0.7.orig/src/Makefile	2016-02-25 13:08:47.394452025 -0500
+++ fits-1.0.7/src/Makefile	2016-02-25 13:10:40.437730825 -0500
@@ -1,4 +1,6 @@
-MKOCT_FLAGS = $(shell pkg-config --libs cfitsio --cflags cfitsio)
+MKOCTFILE ?= mkoctfile
+PKG_CONFIG ?= pkg-config
+MKOCT_FLAGS = $(shell $(PKG_CONFIG) --libs cfitsio --cflags cfitsio)
 
 all: read_fits_image.oct save_fits_image.oct save_fits_image_multi_ext.oct
 
@@ -6,5 +8,5 @@
 	rm -f *.oct *.o
 
 %.oct: %.cc
-	mkoctfile --verbose -Wall $(MKOCT_FLAGS) $<
+	$(MKOCTFILE) --verbose -Wall $(MKOCT_FLAGS) $<