External toolchain C++ cross-compiler fix

External toolchain C++ cross-compiler fix

package/Makefile.in resets CXX to "" in TARGET_CONFIGURE_OPTS if
BR2_GCC_CROSS_CXX is not set to 'y'. However, when using an external
toolchain, BR2_GCC_CROSS_CXX is not set even if the toolchain has a
C++ cross-compiler.

This patch adds a new BR2_GCC_CROSS_CXX option in the external
toolchain configuration menu, so that just like BR2_INET_RPC,
BR2_INET_IPV6 and the others, it can be set according to the external
toolchain configuration.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>