Enable -D_XPG4_2 for compilation on Solaris

Enable -D_XPG4_2 for compilation on Solaris

Solaris' header files to not make necessary macros (like CMSG_SPACE)
available unless told "this is the API level we want" - thus, do so.

This fixes --multihome on OpenSolaris 11 (at least).

(v2: same patch as in 6eaa70e80aea7, reverted in e25d03a4cc0, and now
with correct description)

trac #750

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20161010073931.54469-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg12634.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>