Fixed use of C++11 for Vivado HLS
Fixed use of C++11 for Vivado HLS

Only Linux machines, this can be achieved by using Clang
instead of GCC as Vivado HLS' back end compiler. The
script.tcl file has been adjusted accordingly.

hipacc/hipacc-vivado,hipacc/hipacc-vivado,hipacc/hipacc-vivado