Looks as Alpine not use `glibc` and used [`musl 1.1.23-r3`](https://pkgs.alpinelinux.org/package/edge/main/x86/musl) library ([sources](http://www.musl-libc.org/releases/musl-1.1.23.tar.gz))
I tried search about usage `#ifdef __MUSL__` and found this [response](https://www.openwall.com/lists/musl/2013/03/29/13).
I also looked into `musl` sources and `musl-dev` package and not found `GLIBC` comparability macros.