Add build flags to make CGO optional

[jarifibrahim]

The ZSTD library requires CGO. This commit adds build flags to ignore
ZSTD if CGO is disabled.

To build badger with ZSTD (and CGO) set CGO_ENABLED=1
To build badger without ZSTD (and CGO) set CGO_ENABLED=0

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This change is 

[pullrequest]

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[coveralls]

Coverage decreased (-0.2%) to 77.409% when pulling a572e60 on ibrahim/zstd-cgo into be6402f on master.

[pullrequest]

Just one comment on the file name.

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Reviewed with ❤️ by PullRequest

[martinmr]

other than a minor fix.

Reviewed 3 of 4 files at r1, 1 of 1 files at r2.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @ashish-goswami, @jarifibrahim, @manishrjain, and @pullrequest[bot])

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table/zstd_nocgo.go, line 25 at r2 (raw file):

)

var errZstdCgo = errors.New("zstd compression requires building with cgo enabled")

"zstd compression requires building badger with cgo enabled"

Add a mention of badger, otherwise users of other software that has badger as a dependency might be confused.

[jarifibrahim]

Reviewable status: 3 of 4 files reviewed, 2 unresolved discussions (waiting on @ashish-goswami, @manishrjain, @martinmr, and @pullrequest[bot])

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table/zstd_nocgo.go, line 25 at r2 (raw file):

Previously, martinmr (Martin Martinez Rivera) wrote…

"zstd compression requires building badger with cgo enabled"

Add a mention of badger, otherwise users of other software that has badger as a dependency might be confused.

Thanks! I've changed it.

[manishrjain]

Reviewed 5 of 5 files at r3.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @ashish-goswami, @martinmr, and @pullrequest[bot])

[zanicar]

Unfortunately go get still fails as a result of the gcc dependency. I can't build what I don't have...

[jarifibrahim]

Unfortunately go get still fails as a result of the gcc dependency. I can't build what I don't have...

Oh, right. go get ... would try to build ZSTD by default. Let me see what we can do here.