musl support

[lrvick]

Simple patch to add musl support.

There is no reliable way to detect musl I am aware of, but we can detect glibc which defines _USE_GNU, then we fall back to musl.

This successfully builds on alpine.

LMK if this approach works for you, and I can at least take a look at CI.

Linking #324

[thepowersgang]

Well, it passes CI.
I assume that you've tested it with your own musl setup?

[lrvick]

It works in alpine's musl, though for some reason not with my own build of musl, which is likely a problem on my end I am currently debugging still.

 (50.0% 1r,0w,0b/2t): proc_macro v0.0.0
:0:0 warn:0:Multiple panic_runtime crates loaded - panic_abort-0_0_0 and panic_unwind-0_0_0
/usr/lib/gcc/x86_64-linux-musl/13.2.0/../../../../x86_64-linux-musl/bin/ld: output-1.54.0/libstd.rlib.o: in function `ZRG3cD15std0_0_0_H190003sys4unix5cvt_r2gCfG1c_AH18closureI_open_c_840g':
libstd.rlib.c:(.text.ZRG3cD15std0_0_0_H190003sys4unix5cvt_r2gCfG1c_AH18closureI_open_c_840g+0x41): undefined reference to `open64'
/usr/lib/gcc/x86_64-linux-musl/13.2.0/../../../../x86_64-linux-musl/bin/ld: output-1.54.0/libstd.rlib.o: in function `ZRG4cD15std0_0_0_H190003sys4unix2fs5lstat0g':
libstd.rlib.c:(.text.ZRG4cD15std0_0_0_H190003sys4unix2fs5lstat0g+0x8e): undefined reference to `lstat64'
collect2: error: ld returned 1 exit status
C Compiler failed to execute - error code 256

[lrvick]

Confirmed the above was related to gcc13 removing automatic includes of stdint, which was fixed in later versions of rust. We will need to backport a patch to make rust 1.54 libstd compile with gcc13 but a non issue for this PR.

I have now confirmed builds both in alpine and my own from-scratch musl-based distro with musl 1.2.4 and GCC12.