This is fixed for me now, the test is properly quoted because the shell variable may not exist (one test done conditionally inside another test).

• https://hg.savannah.gnu.org/hgweb/octave/rev/c81b471cdb28

Thanks for catching this error.

I built sundials (4.1.0 now) with
-DKLU_INCLUDE_DIR=install_prefix/include/suitesparse

The suitesparse part does not propagate to the sunlinsol_klu header, so klu.h is not found. I replaced <klu.h> with <suitesparse/klu.h> after installation in the installed sunlinsol_klu.h, and then the build configures and compiles successfully.

Without that edit I get the "unary operator expected" error from the configure script.

I was still at sundials 2.7.1, but dev would not build anymore with https://savannah.gnu.org/bugs/?55911. I upgraded sundials to 3.2.1, and now I can build again, however there are some issues.

checking for sunlinsol/sunlinsol_dense.h... yes
checking whether SUNDIALS IDA includes the SUNLINSOL_DENSE linear solver... yes
checking sunlinsol/sunlinsol_klu.h usability... no
checking sunlinsol/sunlinsol_klu.h presence... no
checking for sunlinsol/sunlinsol_klu.h... no
../octave/configure: line 77091: test: =: unary operator expected
configure: WARNING: SUNDIALS IDA library not configured with SUNLINSOL_KLU or sunlinksol_klu.h is not usable; ode15i and ode15s will not support the sparse Jacobian feature

line 77091 in the configure is the first line of this piece of configure code:

  if test $octave_cv_sundials_sunlinsol_klu = yes \
     && test "x$ac_cv_header_sunlinsol_sunlinsol_klu_h" = xyes; then

$as_echo "#define HAVE_SUNDIALS_SUNLINSOL_KLU 1" >>confdefs.h

  else
    warn_sundials_sunlinsol_klu="SUNDIALS IDA library not configured with SUNLINSOL_KLU or sunlinksol_klu.h is not usable; ode15i and ode15s will not support the sparse Jacobian feature"

  { $as_echo "$as_me:${as_lineno-$LINENO}: WARNING: $warn_sundials_sunlinsol_klu" >&5
$as_echo "$as_me: WARNING: $warn_sundials_sunlinsol_klu" >&2;}


  fi

I don't understand yet why it doesn't detect the sunlinsol_klu stuff since I have in my installation dir:

./include/sunlinsol/sunlinsol_klu.h
./lib/libsundials_sunlinsolklu.so
./lib/libsundials_sunlinsolklu.so.1
./lib/libsundials_sunlinsolklu.so.1.2.1