GCC warning removal for long long

(was #300)

I got below warning message on x86 linux machine :
```
"jsoncpp-0.10.2-p1/include/json/config.h:100: warning: ISO C++ 1998 does not support ‘long long’"
gcc version 4.4.7 20120313 (Red Hat 4.4.7-4) (GCC)
```
In reference to

* #292

**long long** is supported in c++-11; for old compilers i.e `0.y.z` branch this patch should be merged.

Author: ya1gaurav

CMakeLists.txt

@@ -99,7 +99,7 @@ if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall")
 elseif ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
   # using GCC
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -pedantic")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -pedantic -Wno-long-long")
 endif()
 
 IF(JSONCPP_WITH_WARNING_AS_ERROR)