Move constant definitions out embedder.h

[stuartmorgan-g]

Constant values should generally not be defined in headers. As currently written, multiple files including this header will cause link errors as the symbols are redeclared in each file with external linkage.

[stuartmorgan-g]

(If for some reason the value absolutely must be in the header, we probably want a #define; declaring them static would address the link error, but is potentially dangerous in some edge cases.)

[jamesderlin]

(If for some reason the value absolutely must be in the header, we probably want a #define; declaring them static would address the link error, but is potentially dangerous in some edge cases.)

Or use enum.

[GeertJohan]

Since the value itself is part of the API "contract", it may make sense to use enum like other kValues in the header file. If that's possible, since the enum value must still be declared -1 to be compatible?

This would then make it easy to use the value while only including the embedder.h file.

[stuartmorgan-g]

Since the value itself is part of the API "contract"

Chris will need to weigh in on that; it's not clear to me that it is.

[cbracken]

There's no reason at all for the value itself to be in the header; these should definitely have been declared extern initially. Good catch!

The value itself must be < 0. The choice of -1 was arbitrary within that constraint. That said, we should preserve that choice in order to ensure compatibility without forcing a recompile of embedders.

[cbracken]

This'll need a rebase to head to pass presubmit. Other than that, lgtm.

[chinmaygarde]

Can you add a check so that this sort of thing gets caught at build time? We have an embedder_include.c that acts as a sanity check to make sure the header can be included in a C context. I suggest adding an embedder_include2.c and preparing an executable target that gets built in the default rules. The linker will trip on errors like this.