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.

# This file is part of MXE.
# See index.html for further information.

PKG             := qt5
$(PKG)_VERSION   = $(qtbase_VERSION)
$(PKG)_DEPS     := $(patsubst $(TOP_DIR)/src/%.mk,%,\
                        $(shell grep -l 'qtbase_VERSION' \
                                $(TOP_DIR)/src/qt*.mk \
                                --exclude '$(TOP_DIR)/src/qt5.mk'))
$(PKG)_FILE      =
$(PKG)_URL       =
$(PKG)_CHECKSUM := #No checksum

define $(PKG)_UPDATE
    echo 'Warning: Updates are temporarily disabled for package $(PKG).' >&2;
    echo $($(PKG)_VERSION)
endef