Optionally use system provided netcdf

[StefanBruens]

Detect a system wide installed netcdf using pkgconfig and use it, in case --enable-netcdf=system is specified.

Fixes #1542.

[lindsayad]

Per #3390 we will likely want to do some comparison of netcdf and hdf5 versions so that we can error out during configure if incompatible versions are detected ... although admittedly the incompatibility of netcdf 4.6.2 with hdf5 1.12 appears to only be in netcdf testing of itself and not in our uses of it