SPEX: Fix building demo without OpenMP.

[mmuetzel]

Depend on SPEX_HAS_OPENMP (not SPEX_USE_OPENMP) for linking to OpenMP.

This should avoid the configuration error seen, e.g., here:
https://github.com/DrTimothyAldenDavis/SuiteSparse/actions/runs/9201634447/job/25310164131#step:22:73

CMake Error at CMakeLists.txt:495 (target_link_libraries):
-- Generating done (0.1s)
  Target "spex_demo_threaded" links to:

    OpenMP::OpenMP_C

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.



CMake Generate step failed.  Build files cannot be regenerated correctly.
make: *** [demos] Error 1

[mmuetzel]

I triggered the macos runner on my fork of the repository and the SPEX demos build and run successfully with this change even if OpenMP cannot be found:
https://github.com/mmuetzel/SuiteSparse/actions/runs/9203948510/job/25316457587#step:22:337

./build/spex_demo_threaded          f ExampleMats/10teams.mat.txt   ExampleMats/10teams.rhs.txt
spex_demo_threaded: without OpenMP
solving Ax=b with nthreads: 1, with 10 trials per thread
Please wait...
SPEX thread 0: ok.  All systems solved exactly.
SPEX thread test passed