This hooks the existing OpenMP support, as it was already available in
the CLooG backend, to our isl code generator backend.
This is the last feature, that was only available in the CLooG backend and that
blocked the by-default use of the isl backend. As the isl backend is with this
commit not only at feature parity, but has several advantages (run-time
alias checks, delinerization, ...), we aim to switch the default code generation
backend to isl. This switch will take place after a brief in-tree testing period
for the new OpenMP support in our isl backend.
TODO: - Still needs to be documented/style-checked
- We do not use -enable-polly-openmp yet
- We may want to reduce the use of OpenMP specific variable names
Comments are already appreciated.