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 cfitsio.orig/configure.in cfitsio/configure.in
--- cfitsio.orig/configure.in	2019-03-05 10:42:18.453397421 -0500
+++ cfitsio/configure.in	2019-03-05 10:44:29.464686105 -0500
@@ -452,7 +452,7 @@
 else
   AC_CHECK_PROG([CURLCONFIG], [curl-config], [curl-config], [], [], [/opt/local/bin/curl-config])
 fi
-if test "x$CURLCONFIG" != x; then
+if test "x$CURLCONFIG" != x && test "x$CURLCONFIG" != "xnone" ; then
   CURL_LIB=`$CURLCONFIG --libs`
   CURL_INC=`$CURLCONFIG --cflags`
   # Also reject anaconda curl on Macs until further notice: