Closes gh-1344

[oleksandr-pavlyk]

Define CYTHON_EXTERN_C as empty for C compilers.

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• Have you added a test, reproducer or referred to an issue with a reproducer?
• Have you tested your changes locally for CPU and GPU devices?
• Have you made sure that new changes do not introduce compiler warnings?
• Have you checked performance impact of proposed changes?
• If this PR is a work in progress, are you opening the PR as a draft?

[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/1345/index.html

[coveralls]

coverage: 84.95%. remained the same when pulling da63be5 on fix-gh-1344 into 50f1074 on master.

[github-actions]

Array API standard conformance tests for dpctl=0.14.6dev2=py310h7bf5fec_4 ran successfully.
Passed: 913
Failed: 87
Skipped: 119

[oleksandr-pavlyk]

Verified that this change fixes build of numba-dpex. Hence merging. A test to try to build native C extension that includes the "dpctl_capi" header file will be added in a separate PR.

[github-actions]

Deleted rendered PR docs from intelpython.github.com/dpctl, latest should be updated shortly. 🤞

[github-actions]

Array API standard conformance tests for dpctl=0.14.6dev2=py310h7bf5fec_4 ran successfully.
Passed: 913
Failed: 87
Skipped: 119