Build-mode

[dcharkes]

Introduces BuildMode with release and debug values.

This is not an enum, but a collection with two opaque objects. (Similar to Target.) This way adding more values later will not be a breaking change.

Package c_compiler picks up the configuration and sets either the DEBUG or RELEASE macro to 1.

We should probably do the mapping from BuildMode to defines in build.dart files, because it might be project specific. The defines should be added to the CBuilder constructors or run function.

Bug:

• [native_assets_cli] build_mode #50

[dcharkes]

(new commits are not showing up somehow)