Context

Address build issues on mac:

In case CC is already set, we avoid overwriting. On my mac the default clang is /usr/bin/clang rather than the one installed with llvm: /opt/homebrew/opt/llvm/bin/clang.
Add instructions for installing wabt

Manually testing the MR

Follow the instructions from the README for building.

Document the interface of any function added or modified (see the coding guidelines)
Document any change to the user interface, including configuration parameters (see node configuration)
Provide automatic testing (see the testing guide).
For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
Select suitable reviewers using the Reviewers field below.
Select as Assignee the next person who should take action on that MR