autotools: recognize more Linux targets when setting `-D_GNU_SOURCE`

autotools: recognize more Linux targets when setting `-D_GNU_SOURCE`

To set `-D_GNU_SOURCE`.

Required to declare `accept4` on uclibc targets. `host_os` can have
the value `uclinux-uclibcgnueabi` in this case.

Fixing:
```
cf-socket.c: In function 'cf_tcp_accept_connect':
cf-socket.c:2141:18: error: implicit declaration of function 'accept4'; did you mean 'accept'? [-Werror=implicit-function-declaration]
 2141 |     s_accepted = accept4(ctx->sock, (struct sockaddr *) &add, &size,
```

Follow-up to 3d02872be7cfe6dcdef4b02321b47af19e1ce268 #16979

Reported-by: Kadambini Nema
Fixes #17512
Closes #17524