Fix C compilation

[Pilzschaf]

To be able to use ffx_spd in C I put the overload of the SpdSetup with fewer parameters inside a __cplusplus preprocessor block. This way it compiles correctly in C mode and C++ user can still benefit from the simpler additional Setup call. Additionally I had to change the casting syntax at two points to the traditional C casting syntax. This should fix compilation of ffx_spd for C compilers.