[tlbuild] Support for musl

[Henri]

On Sat, 2018-01-27 at 22:14 +0000, Karl Berry wrote:
>     the command line parameter -dumpmachine.
> 
> In terms of using this to determine the TL platform name: the
> information seems sufficient, but the potential problem is requiring a
> compiler to be present on the installing machine. My sense is that
> nowadays many machines do not have a compiler installed.
> 
> It seems like there must be some direct way to determine if musl is
> being used. ldd output? /lib/libc.so being a symlink? Something in
> sysctl or /proc or /something?
> 
> Meanwhile, no problem to avoid the whole issue by installing it via
> -custom-bin ... --thanks, karl.

My patch for config.guess was accepted.

https://git.savannah.gnu.org/cgit/config.git/commit/?id=3d00f60242f1726fc6eaa38e09435a969ee7ebe5

It uses ldd to detect whether musl is used because ldd prints "musl libc" to
stderr.
https://git.musl-libc.org/cgit/musl/tree/ldso/dynlink.c#n1538