Fixes for compiling with a newer (system) jsoncpp (#4429)

* Move included json code to jsoncpp subdirectory

This is needed to avoid having to specify the minetest src directory
as a system include when fixing the json includes.

* Fix json includes

They used "", so that the compiler searches the project's directory
first. The result was that when compiling with a system jsoncpp,
the project's own version of json.h was still included, instead of
the system version.

The includes now use <>, so a system location, or one specified with
'-Ilocation' is searched only.

* Fix for jsoncpp deprecated function warning

When compiling with a newer version of jsoncpp (and
ENABLE_SYSTEM_JSONCPP=true), jsoncpp emits a warning
about a deprecated function that minetest uses.

src/jsoncpp/json/CMakeLists.txt

@@ -0,0 +1,7 @@
+if(MSVC)
+	set(CMAKE_CXX_FLAGS_RELEASE "/MT /O2 /Ob2 /D NDEBUG")
+endif()
+
+add_library(jsoncpp jsoncpp.cpp)
+target_link_libraries(jsoncpp)
+

src/jsoncpp/json/UPDATING

@@ -0,0 +1,16 @@
+#!/bin/sh
+cd ..
+svn co https://jsoncpp.svn.sourceforge.net/svnroot/jsoncpp/trunk/jsoncpp jsoncpp
+svn up jsoncpp
+cd jsoncpp
+python amalgamate.py
+cp -R dist/json ..
+cp dist/jsoncpp.cpp ../json
+
+# maybe you need to patch:
+# src/json/jsoncpp.cpp:
+# -#include <json/json.h>
+# +#include "json/json.h"
+
+#svn export --force https://jsoncpp.svn.sourceforge.net/svnroot/jsoncpp/trunk/jsoncpp/src/lib_json json
+#svn export --force https://jsoncpp.svn.sourceforge.net/svnroot/jsoncpp/trunk/jsoncpp/include/json json