[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.
8 jobs for <span class="ref-name">compilation</span> in 24 minutes and 33 seconds (queued for 1 second)
Status Job ID Name Coverage
  Style
passed #42836
integration
checkpatch

00:00:05

passed #42835
integration
repoquality

00:00:02

 
  Build
passed #42840
make:generic

00:08:47

passed #42841
knl
make:knl

00:06:53

passed #42842
make:out-of-tree

00:08:47

canceled #42837
make:generic

00:00:09

canceled #42839
knl
make:knl

canceled #42838
make:out-of-tree