Merge pull request #14734 from sheenobu/sipsak_fixbuild_gnu89

sipsak: fix build by using --std=gnu89

pkgs/tools/networking/sipsak/default.nix

···
     c-ares
   ];
 
+  NIX_CFLAGS_COMPILE = "--std=gnu89";
+
   src = fetchurl {
     url = "https://github.com/sipwise/sipsak/archive/mr${version}.tar.gz";
     sha256 = "769fe59966b1962b67aa35aad7beb9a2110ebdface36558072a05c6405fb5374";