Fix build warnings

[macdrevx]

Previously, the project had 5 build warnings:

This PR addresses these warnings.

• The first two are addressed by specifying that two enums should use an architecture-dependent size. This fixes the first two warnings when casting from void * to the enum type.
• The last 3 warnings are in generated code. Newer versions of the code generator appear to have resolved these warnings, but for now, I'm just making the casts explicit. If we update the gumbo version in the future, hopefully we'll get regenerated code that also addresses these issues.