gnucobol: fix build with GCC 15 (#475950)

pkgs/by-name/gn/gnucobol/package.nix

@@ -91,9 +91,23 @@ stdenv.mkDerivation (finalAttrs: {
     substituteInPlace configure --replace-fail '"GNU strip"' 'FAKE GNU strip'
   '';
 
-  # error: call to undeclared function 'xmlCleanupParser'
-  # ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
-  env.CFLAGS = lib.optionalString stdenv.hostPlatform.isDarwin "-Wno-error=implicit-function-declaration";
+  # GCC 15 changed some warnings to errors, particularly around function pointer types
+  # (C23 empty parentheses means no args, not unspecified). These flags are needed
+  # until gnucobol is updated to compile cleanly with GCC 15.
+  # See: https://gcc.gnu.org/gcc-15/porting_to.html
+  env.CFLAGS =
+    let
+      # Clang needs -Wno-error=implicit-function-declaration for xmlCleanupParser
+      clangFlags = "-Wno-error=implicit-function-declaration";
+      # GCC 15+ needs additional flags for incompatible pointer type errors
+      gcc15Flags = "-Wno-error=incompatible-pointer-types -std=gnu11";
+    in
+    if stdenv.cc.isGNU && lib.versionAtLeast stdenv.cc.version "15.0.0" then
+      gcc15Flags
+    else if stdenv.cc.isClang then
+      clangFlags
+    else
+      "";
 
   enableParallelBuilding = true;