Compilation is broken with exceptions disabled

[viettrungluu]

Compilation is broken with exceptions disabled (for current HEAD of master).

I get

.../optional_lite/include/nonstd/optional.hpp:1662:22: error: 'nonstd::optional_lite::bad_optional_access' has not been declared
 using optional_lite::bad_optional_access;
                      ^

Presumably https://github.com/martinmoene/optional-lite/blob/4c4da9074ac4c779bb81332fd82292edd8a6089d/include/nonstd/optional.hpp#L1662 should be gated on #if ! optional_CONFIG_NO_EXCEPTIONS.

I'd be happy to send a PR.