Fix compilation with MUSL 1.2.4+

[thomaslaurenson]

MUSL 1.2.4 removed 64 interfaces for some common macros used in Stormlib.

On the API level, the legacy "LFS64" ("large file support") interfaces, which were provided by macros remapping them to their standard names (#define stat64 stat and similar) have been deprecated and are no longer provided under the _GNU_SOURCE feature profile, only under explicit _LARGEFILE64_SOURCE. The latter will also be removed in a future version. Builds broken by this change can be fixed short-term by adding -D_LARGEFILE64_SOURCE to CFLAGS, but should be fixed to use the standard interfaces.

Source: https://musl.libc.org/releases.html

This will need a PR to resolve, as _LARGEFILE64_SOURCE is a temporary workaround which will also be deprecated in the future.

See #63