Add compiler #error directive for GCC retadation

When compiling using GCC on Linux (not other platforms), and using the -std=c99 parameter, the compiler does ```#include <bits/sigaction.h>``` (this is actually an issue with glibc, imo, but what are you gonna do with that steaming pile).  This causes mowgli_signal.c to break hilariously, with the most stupid error messages ever.

This patch adds a much less stupid error message at the top, so people trying to build C99 code (or people using retarded distros that ship "-std=c99" in default CFLAGS) won't fall into the same trap.  Beyond pulling in bits/sigaction.h directly in these cases - which you could just ```s$error.*$include <bits/sigaction.h>$``` in this diff if you want to - there is no other satisfactory solution.

Bug is documented [here](http://forums.debian.net/viewtopic.php?t=6408) on Debian.

Future workarounds that I am too tired to write now could include:
* do not rely on ```sigaction``` being there since it is not "standard" and write something using just ```signal``` (LOL)
* do not allow compiling on glibc with -std=c99 (alpine, master race)
* directly ```#include <bits/sigaction.h>``` for poor saps stuck on glibc