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.

1cab2860069ea9401ff3f28f1fd9654bc777026b release-4-0-0
a047bc8d7910849a3ddcb4d1856ce3dd2b912aa4 octave-4.0.1-binary
e622765c49c736f6e8e4627310a59f1c49fae283 octave-release-4.2.0
ebc418be26ec62519a6ddceb6a6f35b368e49f92 octave-release-4.4.0
7f8ff4800eefbf0ba6c842af4d20c0dc1ae6e89a octave-release-4.4.1
3ef2868263e4ac69b09508acf07255e8fa397afb octave-release-5.1.0
9057112edb8e26e6880cbddabcaa43acaafcc841 octave-release-5.2.0
9057112edb8e26e6880cbddabcaa43acaafcc841 octave-release-5.2.0
8c1e1ef46ed22ddb1e0c1ab94137d631cd97495f octave-release-5.2.0
00a0eac6b6cd816937d25914d371e856d839bcd8 octave-release-5.2.0_1
039563087c70904213cd835dc3e9a8c5025ec6eb octave-rc-6.0.90