pybind11_add_module(): OPT_SIZE argument for size-based optimization

[wjakob]

This PR adds way of switching this specific target over to “optimize size” flags (-Os) no matter what the project itself uses (except in debug mode, where optimizations remain disabled). Performance within bindings is usually not that critical, and this can make a quite substantial difference in projects that are weighted down by heavy (10-100MB) python extension libraries. I’m seeing ~25% size reduction just from that.

The PR also contains two other very minor changes:

1. cmake appends a dummy define -D${target_name}_EXPORTS when compiling shared libraries. These are often not needed, especially not for binding code. I've added directives that remove them.

2. pybind11 prints two redundant lines when included by a parent project (via add_subdirectory), I've deleted one of them.

[henryiii]

Looks good to me!

[henryiii]

Are these harmful, though? I would assume they do nothing if never used, and someone might expect them to be there?

[wjakob]

Ok, fair enough -- I'll delete them again.

[henryiii]

This branch has conflicts that must be resolved

This is why changlog entries in PRs can be a bit of a pain. I've never come up with a reasonable solution, though.

Since both of my merged PRs have affected this, I've rebased for you.

(If you need to update, git fetch && git reset --hard will sync back up - make sure you don't have local changes (if so, leave of the --hard).

[YannickJadoul]

For what it's worth, adding my checkmark :-)

[wjakob]

Merged with some documentation improvements following @YannickJadoul's question. I got rid of the originally proposed change to remove the -D..._EXPORTS flag.