Musl is (of course) not glibc.  Its confstr(3) does not understand
_CS_GNU_LIBC_VERSION.  That's fair.  Problem is, its unistd.h has that
constant defined for unknown reason.  We cannot blindly say the libc is
glibc by looking at it.  Instead we have to kick it, then see if it
quacks like a duck.

See https://git.musl-libc.org/cgit/musl/tree/include/unistd.h