ecl: fix build with gcc15

- add "CFLAGS=-std=gnu17" to `configureFlags`.

There are patches upstream to make it build with `-std=gnu23`
(default in gcc15):
https://gitlab.com/embeddable-common-lisp/ecl/-/merge_requests/325
https://gitlab.com/embeddable-common-lisp/ecl/-/merge_requests/345
but applying them makes it not build with `-std=gnu17` (default in gcc14)
instead.

Fixes build failure with gcc15:
```
/build/ecl-24.5.10/src/c/dpp.c:112:13: error: 'bool' cannot be defined
via 'typedef'
  112 | typedef int bool;
      |             ^~~~
/build/ecl-24.5.10/src/c/dpp.c:112:13: note: 'bool' is a keyword
with '-std=c23' onwards
/build/ecl-24.5.10/src/c/dpp.c:112:1: warning: useless type name in
empty declaration
  112 | typedef int bool;
      | ^~~~~~~
```

pkgs/development/compilers/ecl/default.nix

@@ -59,6 +59,8 @@ stdenv.mkDerivation rec {
   ];
 
   configureFlags = [
+    # Fix build with gcc15
+    "CFLAGS=-std=gnu17"
     (if threadSupport then "--enable-threads" else "--disable-threads")
     "--with-gmp-incdir=${lib.getDev gmp}/include"
     "--with-gmp-libdir=${lib.getLib gmp}/lib"