package/python-greenlet: add missing C++ dependency

Fixes:
 - http://autobuild.buildroot.net/results/529/5291fe0fac1d5c7b6587e4a566991a764a2632fb

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/python-greenlet/Config.in

@@ -12,7 +12,12 @@ config BR2_PACKAGE_PYTHON_GREENLET_ARCH_SUPPORTS
 config BR2_PACKAGE_PYTHON_GREENLET
 	bool "python-greenlet"
 	depends on BR2_PACKAGE_PYTHON_GREENLET_ARCH_SUPPORTS
+	depends on BR2_INSTALL_LIBSTDCPP
 	help
 	  Lightweight in-process concurrent programming.
 
 	  https://github.com/python-greenlet/greenlet
+
+comment "python-greenlet needs a toolchain w/ C++"
+	depends on BR2_PACKAGE_PYTHON_GREENLET_ARCH_SUPPORTS
+	depends on !BR2_INSTALL_LIBSTDCPP