gnucobol: fix build with GCC 15

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

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;