no-eval fallback for cwrap

cwrap returns a dynamically generated function for better performances.

However, in some environments, calling eval is not possible.
So this patch proposes a static version of cwrap when NO_DYNAMIC_EXECUTION is activated.
fix indentation
Add a warning when using slow cwrap

When ASSERTIONS and NO_DYNAMIC_EXECUTION are set, and cwrap is used,
display a warning.
Merge pull request #2976 from lovasoa/patch-3

no-eval fallback for cwrap