Merge pull request #2556 in HDFFV/hdf5 from ~LRKNOX/hdf5_lrk:hdf5_1_8 to hdf5_1_8
* commit 'f80019dfda61ba42c189277dc16643648e23553d':
Update config/gnu-cxxflags.
Update to use optimization flags, C++ error flags, and correct libhdf5.settings.in to remove unworkable changes.
