configure: Support configuring C++ compiler

Add configuration for C++ compiler in configure and Makefiles.
The C++ compiler is choosed as following:
 - ${CXX}, if it is specified.
 - ${cross_prefix}g++, if ${cross_prefix} is specified.
 - Otherwise, c++ is used.

Currently, usage of C++ language is only for access to Windows VSS
using COM+ services in qemu-guest-agent for Windows.

Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Micael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>