CMakeLists.txt: Treat conversion warning as error only with JSONCPP_WITH_WARNING_AS_ERROR=On

Fixes errors when building with buildroot:
http://autobuild.buildroot.net/?reason=jsoncpp-1.7.2

Signed-off-by: Bernd Kuhls <[email protected]>

Author: bkuhls

CMakeLists.txt

@@ -107,11 +107,11 @@ if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall -Wconversion -Wshadow -Werror=conversion -Werror=sign-compare")
 elseif (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
   # using GCC
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall -Wconversion -Wshadow -Wextra -Werror=conversion")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall -Wconversion -Wshadow -Wextra")
   # not yet ready for -Wsign-conversion
 
   if (JSONCPP_WITH_STRICT_ISO AND NOT JSONCPP_WITH_WARNING_AS_ERROR)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pedantic")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror=conversion -pedantic")
   endif ()
 endif()