Replced boost::filesystem::path::native with ::path::string
Replced boost::filesystem::path::native with ::path::string

This allows the code to compile with Visual Studio

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