Fix compilation with GCC 4.4.

It was broken by fe468ac14255f079cd4ad0546d3c51f8e3f8031a.

The *nullptr* is supported by GCC 4.6 and newer, but `__GXX_EXPERIMENTAL_CXX0X__` is already defined in GCC 4.4, so it is not a valid way to check for nullptr availability. Let's just look at the GCC version then.