Avoid manual repetition of suffixes in a make script

[elfring]

A suffix like “cmo” is repeated several times for the definition of two variables. I suggest to avoid this.

• The function “addsuffix” can perform the desired data transformation for you.
• It can also be a bit more efficient to append suffixes at other places instead of replacing them, can't it?

[whitequark]

Efficiency is really not a concern here. I'd be surprised if ocamlbuild spends even a hundred milliseconds in make itself.

[elfring]

Do you like a bit of code reduction more there?

[whitequark]

Yeah, that's a perfectly sensible change.

[gasche]

@elfring would you like to send a pull request?