utility/realname: do not return an empty string

When the variable 'gecos' contained the string ",,,", it returned an empty name,
which causes hdf5 to fail writing the attribute.
doc: add links to testing release 1.0-alpha1
cmake: remove unused VARIANT switches

CELL_SUMMATION_ORDER and HILBERT_ORDER are should always be enabled and
are not used anymore. These switches are a leftover from version 0.1.x.

Update documentation accordingly, include HALMD_POTENTIALS.
doc: update list of supported compilers

Since the switch to C++11, only recent compilers are supported. Remove
older compilers.

I'm not sure whether we want to maintain such a detailed list of
compilers. In particular, I see not much benefit from listing all
possible non-distro versions.
doc/conf.py: replace PROJECT_NAME by PROGRAM_DESC

The long name (PROGRAM_DESC) is used for PR purposes, the abbreviation
(PROJECT_NAME) is rather informal.
doc: set content width to 72em
doc: add cuda package to ubuntu guide
packages.mk: update HDF5 to 1.8.12