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 -uNr a/libgnu/unistd.in.h b/libgnu/unistd.in.h
--- a/libgnu/unistd.in.h	2016-06-18 18:47:32.000000000 -0400
+++ b/libgnu/unistd.in.h	2016-06-23 18:45:42.410781867 -0400
@@ -734,7 +734,7 @@
    Null terminate it if the name is shorter than LEN.
    If the host name is longer than LEN, set errno = EINVAL and return -1.
    Return 0 if successful, otherwise set errno and return -1.  */
-# if @UNISTD_H_HAVE_WINSOCK2_H@
+# if !@HAVE_GETHOSTNAME@ && @UNISTD_H_HAVE_WINSOCK2_H@
 #  if !(defined __cplusplus && defined GNULIB_NAMESPACE)
 #   undef gethostname
 #   define gethostname rpl_gethostname