[Eva] New file engine/function_calls to decide how to analyze a function.

Function [define_analysis_target] decides whether a builtin, the specification
or the definition is used for the analysis of a given C function, according to
Eva parameters.
This function is then used in compute_functions.ml.

[define_analysis_target] also decides whether the resulting states of an
analysis should be saved, according to [-eva-results] parameters.

Finally, [define_analysis_target] registers analyzed function calls in tables,
with the caller/callsite and the defined analysis status.