Some Makefiles were using export to set OCAML_FLEXLINK "globally" while
others set a variable FLEXLINK_ENV and set the environment explicitly.

All Makefiles now use FLEXLINK_ENV and also only invoke it on linking
commands (rather than, for example, all invocations of ocamlopt).

Manual examples

I hit a Lexer error by mistakenly closing an environment with

    \end{caml_example*}{verbatim}

instead of

    \end{caml_example*}

and caml_tex would not quote the wrong input or indicate
at which line the error was, which makes debugging painful.

Reviewed by Sébastien Hinderer.

(no change entry needed)