Do not use min in grid stride loop setup.
Do not use min in grid stride loop setup.

This avoids problems with min on MacOsX (see #92)

ddemidov/vexcl,Poulpy21/vexcl,kapadia/vexcl,Poulpy21/vexcl,Poulpy21/vexcl,kapadia/vexcl,ddemidov/vexcl