Stop the inclusion of system libraries in the HDF5_LDFLAGS macro.
HDF5 h5cc -showconfig likes to include -L/usr/lib[64] which is an issue when other libraries we want to use (mostly GSL) are also installed on the system, and we can end up using those instead of the ones we want. Hack the HDF5 m4 macro to stop this. Horrible, but libtool likes to reorder libraries so we cannot rely on ordering alone.
Fixes #783 (closed), also another attempt at !1477 (merged)