Auto detect SMALL_DISABLE_EXCEPTIONS

[alandefreitas]

The problem

We should be able to detect whether the compiler supports exceptions whenever the user doesn't explicitly set SMALL_DISABLE_EXCEPTIONS.

Proposed solution

Use the same parameters as in Boost.config to detect that.

Alternatives I've considered

A simple macro the user needs to set manually. Most people will simply forget to do that.

Additional context

Expecting users to set SMALL_DISABLE_EXCEPTIONS manually doesn't work well in most cases.