libxc: portability fixes for NetBSD

libxc: portability fixes for NetBSD

Attached patch makes libxc build again on NetBSD after the recent rework.

[ Modified by iwj:

I changed the name of the new make variable from LIBDL to DLOPEN_LIBS.
The latter conforms to the naming scheme for similar variables found
in config/*.mk - PTHREAD_LIBS et al.

Also I moved the setting of the variable to -dl from Linux to StdGNU
(which makes it apply more widely) and also added it to SunOS.mk
(based on pure guesswork). ]

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>