pigpio: fix build with modern GCC

With C23, the `pigpio` is currently broken because of various
underdefined function pointer argument lists. The upstream
code hasn't been patched to fix this yet (as of today, the
latest release is still v79 from 2021). Switching the build
to `-std=gnu17` fixes this for now.

pkgs/by-name/pi/pigpio/package.nix

@@ -26,6 +26,8 @@ mkDerivation rec {
     hash = "sha256-Z+SwUlBbtWtnbjTe0IghR3gIKS43ZziN0amYtmXy7HE=";
   };
 
+  NIX_CFLAGS_COMPILE = "-std=gnu17";
+
   nativeBuildInputs = [
     cmake
   ];