jsoncpp: don't build tests

Fixes:
http://autobuild.buildroot.net/results/a0f/a0f60a1df243bdae875f12668011a53a70071ee6/
http://autobuild.buildroot.net/results/f88/f8885520b5892d49164c88a974e7dffc1c3c2ce0/
http://autobuild.buildroot.net/results/448/448745830f58496e6dcdf48e2ad57650b4744909/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/jsoncpp/jsoncpp.mk

@@ -11,6 +11,7 @@ JSONCPP_LICENSE_FILES = LICENSE
 JSONCPP_INSTALL_STAGING = YES
 
 JSONCPP_CONF_OPTS += \
-	-DJSONCPP_WITH_POST_BUILD_UNITTEST=OFF
+	-DJSONCPP_WITH_POST_BUILD_UNITTEST=OFF \
+	-DJSONCPP_WITH_TESTS=OFF
 
 $(eval $(cmake-package))