Pass openmp flag to linker

[akshayka]

On Ubuntu 20.04, if -fopenmp is passed when compiling but not when linking, importing diffcp results in the following error

>>> import diffcp
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/akshay/src/cvxgrp/diffcp/diffcp/__init__.py", line 3, in
<module>
    from diffcp.cone_program import solve_and_derivative, \
  File "/home/akshay/src/cvxgrp/diffcp/diffcp/cone_program.py", line 3,
in <module>
    import diffcp.cones as cone_lib
  File "/home/akshay/src/cvxgrp/diffcp/diffcp/cones.py", line 6, in
<module>
    from _diffcp import dprojection, project_exp_cone, Cone, ConeType
ImportError:
/home/akshay/envs/diffcp/lib/python3.8/site-packages/_diffcp.cpython-38-x86_64-linux-gnu.so:
undefined symbol: omp_get_thread_num