Fix variable CC content + add test

[voodoos]

History

• Since Directly call the C compiler  #3565 ocaml_cflags were removed from the :standard set of flags to prevent flags duplication as these are always prepended to the C compiler arguments. However this is not a backward compatible change: an example of that is that it changes the content of variable %{cc}.
• I proposed [Fix] Restore variable %{cc} content (changed by #3565 ) #3686 to restore compatibility but it was not exactly the same either: the new CC content would not be overwritten by cflags from an env stanza which do not include :standard.

This PR

• This PR takes a much simpler approach: adding back the flags in :standard (which will provoke duplication again).
• I also added tests illustrating the interactions between the CC variable and cflags defined in Env stanzas.

Future work

An RFC to propose a streamlining and maybe redefine the role of the CC variable.

[ghost]

Looks good!

Nicely written test BTW, was nice and easy to read :)