Work around C++11 nullptr

[saper]

Provide CXX11_NULLPTR autoconf macro

[coveralls]

Coverage remained the same at 80.4% when pulling 9669f7d on saper:nullptr into 091010d on sass:master.

[xzyfer]

@saper is this still relevant? If so please rebase off master.

[xzyfer]

Ping ^

[saper]

Rebased for @drewwells for experiments with limited compiler environment (Android, MacOS X 10.6 SDK, etc.)

[mgreter]

IMO it's easier to just replace nullptr with 0, I see no real need for it, we use 0 all over the place.

[xzyfer]

I think agree with @mgreter. We don't have the issues nullptr is intended to fix.

[saper]

I think agree with @mgreter. We don't have the issues `nullptr` is intended to fix.

Let's use at least NULL instead :)

[xzyfer]

Tracking in #1657