Remove libcheck from the dependencies.

[iphydf]

We're not gaining much from this library, and it's a burden, especially
for windows development.

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This change is 

[nurupo]

Wow, libcheck looks kind of useless after reading this PR.
Are you sure it doesn't do more things behind the scenes?
Does this work with autotools just fine?

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Review status: 0 of 28 files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable

[iphydf]

What libcheck does behind the scenes:

• If your test crashes (e.g. segfault), it tells you which assertion has been seen passing last, giving you a clue as to where the crash might have been. I.e. it's roughly doing printf debugging. gdb is a lot better at telling you where the crash happened.
• If your test crashes (e.g. segfault), on Linux it tells you that it segfaulted and fails the test rather than aborting the whole process. On windows, it still aborts the whole process.

That's it.

[iphydf]

I'm running a travis build with all the "if:"s commented out, so we can see if this works on all the builds.

[iphydf]

https://travis-ci.org/TokTok/c-toxcore/builds/338134142 - it works on all builds.

[robinlinden]

(the check removal. Haven't reviewed the crypto_memcmp test commit.)

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Reviewed 27 of 27 files at r2.
Review status: 27 of 32 files reviewed at latest revision, 1 unresolved discussion, some commit checks failed.

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CMakeLists.txt, line 224 at r2 (raw file):

target_link_modules(toxnetwork toxcrypto)

message("CMAKE_THREAD_LIBS_INIT: ${CMAKE_THREAD_LIBS_INIT}")

Where did this come from?

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Comments from Reviewable

[robinlinden]

Reviewed 8 of 8 files at r1.
Review status: all files reviewed at latest revision, 1 unresolved discussion, some commit checks failed.

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Comments from Reviewable