Do not require C99 mode for older GCC

[jonahbeckford]

This PR avoids the following error on an older GCC 4.8.5:

#=== ERROR while compiling dune.3.15.0 ========================================#
# context     2.2.0~alpha0~20221228 | linux/x86_64 |  | git+https://github.com/ocaml/opam-repository.git
# path        /work/.ci/o/2.1.1/.opam-switch/build/dune.3.15.0
# command     /work/.ci/o/2.1.1/bin/ocaml boot/bootstrap.ml -j 1
# exit-code   2
# env-file    /work/.ci/o/log/dune-11998-d28a2a.env
# output-file /work/.ci/o/log/dune-11998-d28a2a.out
### output ###
# ocamlc -output-complete-exe -w -24 -g -o .duneboot.exe -I boot unix.cma boot/libs.ml boot/duneboot.ml
# ./.duneboot.exe -j 1
# cd _boot && /work/.ci/o/2.1.1/bin/ocamlopt.opt -c -g -I +threads spawn_stubs.c
# vendor/spawn/src/spawn_stubs.c: In function 'init_spawn_info':
# vendor/spawn/src/spawn_stubs.c:526:5: error: 'for' loop initial declarations are only allowed in C99 mode
# vendor/spawn/src/spawn_stubs.c:526:5: note: use option -std=c99 or -std=gnu99 to compile your code

OP: ocaml/dune#10423