OpenMP and pthreads configuration do not add CXXFLAGS

[holke]

The configure setting --enable-openmp and --enable-pthread do not add the appropriate flags (i.e. -fopenmp) to the CXXFLAGS variable, they only add to the CFLAGS variable.

The corresponding settings come from the sc library and maybe should get fixed there.
Or we implement our own openmp and pthread configure settings and overwrite those of sc.

The current workaround is to set the CXXFLAGS manually:

configure --enable-openmp CXXFLAGS="-fopenmp"