configure.ac: fix calls to mkoctfile for Octave configuration

- As of Octave 5.1.0, mkoctfile no longer works with 'env -'
  (no environment), so need to pass at least PATH and
  LD_LIBRARY_PATH for it to work.
- Still need to clear environment so that it doesn't override
  mkoctfile defined variables, e.g. CXXFLAGS.
1 file changed