package/libvirt: fix libvirtd dependency

Commit 4dff1be05e90 (package/libvirt: libvirtd needs C++ for nmap-ncat)
introduce a recursive dependency (really: a circular dependency):

    package/busybox/Config.in:33:error: recursive dependency detected!
    package/busybox/Config.in:33:   symbol BR2_PACKAGE_BUSYBOX_SHOW_OTHERS is selected by BR2_PACKAGE_EBTABLES_UTILS_SAVE
    package/ebtables/Config.in:11:  symbol BR2_PACKAGE_EBTABLES_UTILS_SAVE depends on BR2_PACKAGE_EBTABLES
    package/ebtables/Config.in:1:   symbol BR2_PACKAGE_EBTABLES is selected by BR2_PACKAGE_LIBVIRT_DAEMON
    package/libvirt/Config.in:44:   symbol BR2_PACKAGE_LIBVIRT_DAEMON depends on BR2_PACKAGE_NETCAT_OPENBSD
    package/netcat-openbsd/Config.in:1:     symbol BR2_PACKAGE_NETCAT_OPENBSD depends on BR2_PACKAGE_BUSYBOX_SHOW_OTHERS

We can't drop the C++ dependency and switch the netcat-openbsd and
nmap-ncat dependencies conditions without adding a glibc dependency.

So always mandate C++ even if is only needed by nmap and not
netcat-openbsd

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit a17c456b2ccd44566c2395efc90d403cd27a2902)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libvirt/Config.in

@@ -44,7 +44,7 @@ if BR2_PACKAGE_LIBVIRT
 config BR2_PACKAGE_LIBVIRT_DAEMON
 	bool "libvirtd"
 	default y
-	depends on BR2_INSTALL_LIBSTDCPP || BR2_PACKAGE_NETCAT_OPENBSD # nmap
+	depends on BR2_INSTALL_LIBSTDCPP # nmap
 	select BR2_PACKAGE_BUSYBOX_SHOW_OTHERS
 	select BR2_PACKAGE_DNSMASQ
 	select BR2_PACKAGE_EBTABLES
@@ -58,6 +58,9 @@ config BR2_PACKAGE_LIBVIRT_DAEMON
 	  Build the libvirt daemon (libvirtd) otherwise build only the
 	  utility programs.
 
+comment "libvirtd needs a toolchain w/ C++"
+	depends on !BR2_INSTALL_LIBSTDCPP
+
 # Stateful drivers are useful only when building the daemon.
 if BR2_PACKAGE_LIBVIRT_DAEMON