superbuild: add API to test that Linux libraries are loadable

Also brings in a static build for netcdf.

* Ben Boeckel: ldd-testing-exclude-fix (common-superbuild!618)
* Ben Boeckel: test-loadable-modules (common-superbuild!615)
* Ben Boeckel: fix_gitattributes (common-superbuild!616)
* Mathieu Westphal: netcdf_fix_build (common-superbuild!614)