LuaWrapper: Disable maybe uninitialized warnings with boost optional

[rgacogne]

Short description

GCC enables -Wmaybe-uninitialized by default with -Wall, and it reports what looks like false positives with boost::optional types.

See:

• https://svn.boost.org/trac10/ticket/12513
• https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78044

I'm not a huge fan of hiding warnings, but these are flooding my terminal causing me to miss other, legitimate, warnings, and they look like false positives to me.
clang does not implement the warning and actually issue a warning if you try to hide a non-existent warning, so I had to exclude it.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)