Move preprocessor definitions to configuration header

[fknorr]

Celerity's list of compile-time feature flags and options has grown considerably over time, and moving them from the command line to a generated header file will keep them more manageable.

Unfortunately we already have the canonical name config.h in use for the run-time configuration, so instead of causing confusion with a second file of the same name, I've chosen to move them into our existing generated header version.h. Not perfect for discoerability, but still semantically meaningful as that file enumerates all config options for the particular version of Celerity that's being built.

This PR also replaces the somewhat ad-hoc SYCL impl detection macros with a uniform set of CELERITY_SYCL_IS_* defines.

Also I've made sure all of our own defines are either 0 or 1, never undefined, and added -Wundef - this should avoid all future confusion around #if vs #if defined .

[github-actions]

Check-perf-impact results: (5b2139f73fb4c21b4bcab6e559cd8c5a)

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Please re-run the microbenchmarks and include the results if your commit could potentially affect performance.

[coveralls]

Pull Request Test Coverage Report for Build 10325268562

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 95.097%

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Totals
Change from base Build 10325124557:	0.01%
Covered Lines:	6624
Relevant Lines:	6722

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 10325210986

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 95.097%

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Totals
Change from base Build 10325124557:	0.01%
Covered Lines:	6624
Relevant Lines:	6722

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💛 - Coveralls