Use -fpermissive build option

The following code doesn't compile on GCC, but does on Clang:

	class color;

	class foo
	{
		color color;
	};

This is because "color" now has a different meaning inside the
class "foo". According to the standard, it's forbidden, but no
diagnostic is required for a violation of the rule.

I don't think that this should be an error, so I'm downgrading it
to a warning with the -fpermissive flag.