opendap test fails on macos x unless system libcurl is linked

I recently enabled the opendap tests which exercise the curl library linked by netcdf-c.

These tests fail on macosx.

Linking the system libcurl instead of the curl built from source fixes this.

@matthew-brett: is there any reason not to use the system libcurl?  Does it make the wheels less portable to other versions of macos (other than the version the wheel is built on)?