[tlbuild] Support for musl

[Reinhard Kotucha]

On 2018-01-30 at 12:12:54 +1300, Henri wrote:

 > My patch for config.guess was accepted.

Congratulations!

 > It uses ldd to detect whether musl is used because ldd prints "musl
 > libc" to stderr.

What does config.guess return?

Regards,
  Reinhard

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Reinhard Kotucha                            Phone: +49-511-3373112
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