Allow an argument to `get_pkg_config`

[mattip]

The wheel can be used in two scenarios:

• in a project which will depend on import scipy_openblas64 to load the shared object into the executable and make the symbols available without the runtime loader needing to find the shared object
• in a project which will not depend on the wheel at runtime, so it will have to ship the shared object and it can be loaded at runtime via a NEEDED directive in the shared object that consumes the symbols

The second method is the one currently used by NumPy/SciPy. The difference between the two is

• The first options requires something like the _distributor_init.py trick to call import scipy_openblas at runtime. It also requires that the scipy-openblas.pcfile have an emptyLibs:` line (on posix).
• The second option will use something like auditwheel to pack the shared object into the project wheel, so it needs a proper Libs: -L${libdir} -l scipy_openblas64_ line in the scipy-openblas.pc file

I think this can be resolved by adding an argument to get_pkg_config() to specify which line to create. Any good thoughts for what that argument should be called?

[mattip]

xref numpy/numpy#25732 (comment)

[mattip]

full_libs?

[rgommers]

NumPy/SciPy use the second option, so that should be the default behavior I think. To avoid the -lscipy_openblas64_ line, maybe something like use_preloading?