Make __attribute__ conditional in all installed headers

Make __attribute__ conditional in all installed headers

__attribute__ is a GNU extension. If we want to link against the
libraries using a different compiler, it needs to be disabled. It was
already disabled in libdw.h, and this patch extends this to the other
headers. We move the defines to libelf.h as that is included in all
the others.

Signed-off-by: Ulf Hermann <ulf.hermann@qt.io>