add option --enable-werror to configure.

Enabling this option turns warnings into errors. By default this option is off.
Make ROOT 6 Beta 3 (5.99.06).
When variadic return stmt doesn't have a return value, invalidate the calculation of the last expr for evaluation.

In the same cases we still need to synthesize the setValueNoAlloc, to initialize
the cling::Value to void.
Protection added against invalid use of the option PIE
Run caching before using the cached vars.
Add support for long double args.