[tlbuild] Support for musl

[Reinhard Kotucha]

On 2018-01-23 at 10:03:24 +0900, Norbert Preining wrote:
 
 > But our TL internal scripts will probably not work, because I guess
 > we cannot easily distinguish between the version of libc.

Hi Norbert,
I expect the C runtime lib in /lib{,32,64}/ and suppose that it's
either called libc* or musl*.

Checking whether libc is used is difficult.  On CentOS there is

  /lib/libc-2.17.so
  /lib/libc.so.6 -> libc-2.17.so

but on Raspbian there is only

  /lib/klibc-YL2Pal4e_FwRI58JJ6S97Xf241g.so

On the other hand it's easy to check whether musl is used or not
because only one Linux distribution so far is using it.  Thus, as a
last resort, we (or autoconf) can at least evaluate the output of
/etc/os-release.

Regards,
  Reinhard

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