Change when using gcc48

[Shane Ambler]

As the port maintainer of graphics/openimageio I have come across a
change when building with gcc48.

The current version of openimageio compiles fine with clang gcc and
gcc46, but when compiled with gcc48 the unlink function is not defined.

The simple solution is to add #include <unistd.h> to the source file
but why is this only needed for gcc48? Is this an intended change?