Explicitly check if compiler supports C99

[jin-eld]

c-tocxocre will start using C99 code, so check if the compiler supports
it and abort in configure with an error if it does not.

closes #413

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[jin-eld]

@robinlinden currently I am explicitly checking for C99, it's also possible to check if the compiler supports "standard ISO C", which at this time happens to be C99, so it's more a future question if we want to enable the latest standard ISO C automatically or if we stick with C99 and move forward only when needed.

[GrayHatter]

https://travis-ci.org/TokTok/c-toxcore/jobs/190539792#L860-L861

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[robinlinden]

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Reviewed 1 of 1 files at r1.
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[robinlinden]

@jin-eld I think explicitly checking for C99 is better than checking if the compiler supports the latest standard ISO C.

[iphydf]

Does this also set the -std=c99 flag?

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[iphydf]

AC_PROG_CC_C99
Ok, found that it does.

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[nurupo]

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