package/cryptopp: needs C++

Fix the following build failure raised since commit
bf4cf9de833e2d571d98d1ca935ea08a3468e463:

no -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -O2 -g0 -D_FORTIFY_SOURCE=1 -fPIC -DCRYPTOPP_DISABLE_ARM_NEON -c cryptlib.cpp
make[1]: no: Command not found

Fixes:
 - http://autobuild.buildroot.org/results/fa90b416a219a512a482e7dc63e2d72b184307c7

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/cryptopp/Config.in

@@ -1,9 +1,11 @@
 config BR2_PACKAGE_CRYPTOPP
 	bool "cryptopp"
+	depends on BR2_INSTALL_LIBSTDCPP
 	depends on !BR2_STATIC_LIBS
 	depends on BR2_USE_WCHAR
 	help
 	  A free C++ class library of cryptographic schemes
 
-comment "cryptopp needs a toolchain w/ dynamic library, wchar"
-	depends on BR2_STATIC_LIBS || !BR2_USE_WCHAR
+comment "cryptopp needs a toolchain w/ C++, dynamic library, wchar"
+	depends on !BR2_INSTALL_LIBSTDCPP || BR2_STATIC_LIBS || \
+		!BR2_USE_WCHAR