Added '--enable-dynamic-loading' to autotools-based builds, and
-DENABLE_DYNAMIC_LOADING to cmake-based builds.

This will allow for compatibility with hdf5 1.8.11 builds that
have enabled dynamic building (depends on libdl).

See Jira ticket NCF-258



git-svn-id: https://sub.unidata.ucar.edu/netcdf/trunk@3266 2e3060e4-1ed8-cf1f-9cc2-9586e844a556