If OpenMP is available (and not disabled at configure time), it is used to build a libgsl-omp library (with a gsl-omp pkg-config package)

This library allows one to use gsl_integration_*_omp variant that will evaluate the integrand in parallel in OpenMP tasks.

Notes:
- gsl_integration_*_omp routines are put in a separate library to avoid link dependencies for programs that do not want to use these new OpenMP routines
- code and tests are derived from the sequential version to avoid code duplication