guile_1_8, guile_2_0: set std=gnu17 to fix build with gcc15 (#476723)

pkgs/development/interpreters/guile/1.8.nix

@@ -65,6 +65,10 @@ stdenv.mkDerivation rec {
     ./CVE-2016-8605.patch
   ];
 
+  env = {
+    NIX_CFLAGS_COMPILE = "-std=gnu17";
+  };
+
   preBuild = ''
     sed -e '/lt_dlinit/a  lt_dladdsearchdir("'$out/lib'");' -i libguile/dynl.c
   '';

pkgs/development/interpreters/guile/2.0.nix

@@ -116,6 +116,10 @@ builder rec {
     "--without-threads"
   ];
 
+  env = {
+    NIX_CFLAGS_COMPILE = "-std=gnu17";
+  };
+
   postInstall = ''
     wrapProgram $out/bin/guile-snarf --prefix PATH : "${gawk}/bin"
   ''