Automatically disable extended float support for broken HDF5

HDF5 < 1.8.12 has a bug that prevents correct identification of the long double data type when the code is built with gcc 4.8.

See also: http://hdf-forum.184993.n3.nabble.com/Issues-with-H5T-NATIVE-LDOUBLE-tt4026450.html

This PR automatically disable extended float support if a buggy version of HDF5 is detected.