Fix -fPIC OCaml compilation error

When compiling C stubs, you may encounter the following issue

warning: relocation against `Caml_state' in read-only section `.text'
relocation R_X86_64_PC32 against undefined symbol `Caml_state' can not be used when making a shared object; recompile with -fPIC

Check if the default C flags of ocamlopt are given. In the field foreign_stubs, check if you give :standard as flags. It will include the default flags given by ocamlc -config. For 4.14.1, the default values are:

$ ocamlc -config
[...]
ocamlc_cflags: -O2 -fno-strict-aliasing -fwrapv -pthread -fPIC
[...]
ocamlopt_cflags: -O2 -fno-strict-aliasing -fwrapv -pthread -fPIC
[...]

You can also simply add -fPIC in the c_flags, but it is better to follow the standard flags included by the OCaml compiler.