rapidjson: needs C++ support

Fixes:

  http://autobuild.buildroot.net/results/ac989c119e4ea90b8318795fe9cf8c3e9395a354/

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

package/rapidjson/Config.in

@@ -1,6 +1,10 @@
 config BR2_PACKAGE_RAPIDJSON
 	bool "rapidjson"
+	depends on BR2_INSTALL_LIBSTDCPP
 	help
 	  A fast JSON parser/generator for C++ with both SAX/DOM style API.
 
 	  http://rapidjson.org/
+
+comment "rapidjson needs a toolchain w/ C++"
+	depends on !BR2_INSTALL_LIBSTDCPP