sparseqr/sparseqr_gen.py : add typical location for SuiteSparseQR_C.h

[SmithB]

On environments that use conda, the suitesparse include directory will often be under the user's conda prefix. We can find it with: include_dirs.append( join(os.environ['CONDA_PREFIX'], 'include', 'suitesparse'))

I added some logic to make sure there's a 'CONDA_PREFIX' in the environment.

[yig]

Looks good to me