Fix hypre_prefix and build_hypre options

[zhanggy2004]

Thank you for developing and maintaining this nice project.

I have been using prebuilt hypre from conda forge, and found the installation process for PyMFEM still builds hypre from source. I am trying to fix this issue in this PR.

Also, when --with-parallel, --with-gslib and --hypre-prefix are all enabled, I found gslib will not be built for the parallel version. This PR tries to fix that as well.

There might be a similar issue in

PyMFEM/_build_system/build_config.py

Lines 328 to 329 in 5fcbd6d

	hypre_prefix = bglb.mfem_prefix
	metis_prefix = bglb.mfem_prefix

, but I have not tested that and will leave it unchanged.

Please let me know your thoughts/suggestions. Thanks.

[sshiraiwa]

@zhanggy2004 Thank you so much for finding this.
@justinlaughlin When you have time, can you take a look? SI don''t use conda to install stuffs. You would be better person to test it.