[fix] Clean-up out-of-tree building

Adapted to the new master from a patch sent by @cfoyer

    When building out of tree as the, make sure that target refer to
    relative paths.

    This commit also clean-up the usage of the flags and defines
    a per-target definition of flags (can be changed for a
    AM_CPPFLAGS if the global definition is good enough).

    Signed-off-by: Clement Foyer <cfoyer@cray.com>

Also adds Clement to the authors list.
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