CSDS: fix missing -fopenmp for csds writer compilation

I'm starting to use the CSDS. First I'm testing it. To compile it, I added the missing -fopenmp. Then I tested the examples/HydroTests/SedovBlast_3D, which works.

It should compile with ./configure --enable-csds. However, I can only compile with --enable-compiler-warnings because of some warnings. I don't know yet if it's only me, since my gcc compiler can show warnings that do not appear to others.