gpm: Fix build with gcc=15

pkgs/servers/gpm/default.nix

@@ -64,6 +64,9 @@ stdenv.mkDerivation {
     "--sysconfdir=/etc"
     "--localstatedir=/var"
     (if withNcurses then "--with-curses" else "--without-curses")
+    # The code won't compile in c23 mode.
+    # https://gcc.gnu.org/gcc-15/porting_to.html#c23-fn-decls-without-parameters
+    "CFLAGS=-std=gnu17"
   ];
 
   enableParallelBuilding = true;