build: shasum vs sha1sum etc.

[brody4hire]

https://stackoverflow.com/questions/28490500/how-to-installing-sha1sum-in-mac-os

Makefile seems to want sha1sum but macOS has shasum. I think we should use an environmental variable for this.

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P.S. It looks like upstream sql-js is now using both sha1sum & sha3sum

[brody4hire]

NOW REVERTED: I just pushed d19fd9b with overridable SHA_SUM

to build on macOS, for example:

make dist/sql-asm-memory-growth.js SHA_SUM=shasum

I would like to get overridable setting SHASUM or SHA1SUM in upstream sql-js version.

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P.S. Should probably revert this, they are now using both SHA-1 & SHA-3 checksums.