fix: Replace SIGUNUSED with SIGSYS and use _GNU_SOURCE

[galmyk]

The SIGUNUSED define was removed from glibc version 2.26 in August 2017. So Termistor cannot be built on GNU/Linux distributions that use glibc>=2.26, such as Fedora>=27 and Ubuntu>=17.10.

From the signal(7) man page:

       SIGUNUSED      -        Core    Synonymous with SIGSYS

After changing SIGUNSED to SIGSYS, there are other problems raise while building the code:

• grantpt, unlockpt and ptsname functions, need _XOPEN_SOURCE >= 500 feature test macro.
• posix_openpt function needs _XOPEN_SOURCE >= 600 feature test macro.
• pipe2 function needs _GNU_SOURCE feature test macro.

Since _GNU_SOURCE implies nearly all feature test macros, adding _GNU_SOURCE is sufficient.

[galmyk]

Anyway, apparently this repo hasn't been updated in several years. I didn't realize this when I submitted this patch and the other one I submitted.