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 octave-sockets-1.2.0.orig/src/Makefile octave-sockets-1.2.0/src/Makefile
--- octave-sockets-1.2.0.orig/src/Makefile	2015-03-20 10:24:40.824141293 -0400
+++ octave-sockets-1.2.0/src/Makefile	2015-03-20 11:31:54.325799984 -0400
@@ -8,12 +8,15 @@
 SRC := $(OCT:.oct=.cc)
 
 MKOCTFILE ?= mkoctfile -Wall
+OCTAVE_CONFIG ?= octave-config
 OCTAVE ?= octave
 
+CANONICAL_HOST_TYPE := $(shell $(OCTAVE_CONFIG) -p CANONICAL_HOST_TYPE))
+
 #The following is necessary to get the sockets package working in Windows.
 #It has been tried on Win7 and XP, in Octave 3.8.0 using mxe-octave (mingw)
 EXTRALIBS := 
-ifeq ($(OS),Windows_NT)
+ifneq (,$(findstring mingw,$(CANONICAL_HOST_TYPE)))
   EXTRALIBS := -lws2_32
 endif