Musl libc fixes

[lemmi]

I was just packaging compsize and found it won't build for musl.

Musl is a little more picky about what includes you need, so I added the missing signal.h. Other than that I think the use of __O_LARGEFILE is unnecessary and discuraged and using #define _FILE_OFFSET_BITS 64 ist the prefered way to do things, even on glibc (see man 2 open).

I confirmed these changes to work with x86_64-musl.

[kilobyte]

signal.h is obvious, thanks.

WRT __O_LARGEFILE: it works a bit differently than _FILE_OFFSET_BITS 64: the former is a flag to open() while the latter redefines open as open64() — a different syscall. And as I already had the latter, the flag is indeed redundant.

[kilobyte]

@lemmi: would a tag make your life easier?

[lemmi]

@kilobyte yes it would, thx :)