Adding support for the MUSL C library

[Niels Möller]

Josh Conner <joshconner at google.com> writes:

> When trying to configure GMP for use with the MUSL libc, it failed to
> detect that FILE is defined. Below is a diff that fixes this - I
> thought I would pass it along so it might be incorporated. 

Thanks.

> Is there anything I can do to help get this committed?

Note that gmp-5.1.3 is quite old. But I think these checks in gmp-h.in
are identical in the current version.

For more substantive changes we ask for an FSF copyright assignment.
(Which I believe is already in place with google. And feel free to mail
me at nisse at google.com if you have any company-specific questions).

Regards,
/Niels

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