Changes

core: fix a #if vs #ifdef for constructor attribute support

Unnoticed until now because these lines were added recently,
way after all compilers added support for this attribute...
until somebody tried autoconf with MSVC.

Thanks to Alexander Neumann for the report.

Closes #485

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

core: -no-undefined is now supported on Windows

And required by libtool.

Closes #486

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

private/misc.h: use HWLOC_HAVE_ATTRIBUTE_UNUSED instead of __HWLOC_HAVE_ATTRIBUTE_UNUSED

The latter is defined in public headers depending on whether private
headers are available or not.
The formet is the official internal thing.

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

cpukinds: don't duplicate an empty array of cpukinds

Fixes a segfault during dup() on AIX.

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>