Fix VEX_STRINGIZE_SOURCE
Fix VEX_STRINGIZE_SOURCE

In some cases BOOST_PP_STRINGIZE (used in the background of
VEX_STRINGIZE_SOURCE) did not work.

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