Use OCTAVE_HOME to determine octave root dir.

The keyword PREFIX is no longer supported by octave-config and
OCTAVE_HOME is to be used in its stead. See
https://hg.savannah.gnu.org/hgweb/octave/rev/b95c430c0649

cmake/FindOctave.cmake

@@ -68,7 +68,7 @@ find_program(OCTAVE_CONFIG_EXECUTABLE
 
 
 if (OCTAVE_CONFIG_EXECUTABLE)
-  execute_process (COMMAND ${OCTAVE_CONFIG_EXECUTABLE} -p PREFIX
+  execute_process (COMMAND ${OCTAVE_CONFIG_EXECUTABLE} -p OCTAVE_HOME
                     OUTPUT_VARIABLE OCTAVE_ROOT_DIR
                     OUTPUT_STRIP_TRAILING_WHITESPACE)