Updates to autotools

Adds new function git_branch() to version.c and support to set it.
Adds --enable-compiler--warnings (error/yes/no), error is default.
Add support for clang sanitizer flags.
Fix small issues in units.c, once warnings become errors.
Make HDF5 compulsory and stop configure if not found.



See merge request !36

Former-commit-id: 74ba76d4cb1eac483283e29394235f36d189f03e