CMake: Add include directory to jsoncpp_lib target

[Gachapen]

This is needed if using the jsoncpp CMake project as a subproject of another application. The application CMake project will only need to link with the jsoncpp_lib target and the include directory will follow. Without this the application project would need to manually include the json include directory.

[cdunn2001]

CMake Error at src/lib_json/CMakeLists.txt:46 (TARGET_INCLUDE_DIRECTORIES):
  Unknown CMake command "TARGET_INCLUDE_DIRECTORIES".

[Gachapen]

I don't understand why it can't find the command. Is it a problem with the build bot? The exact same command is used inside the if test below at line 48 (this was there from earlier).

[Gachapen]

I did some more research and it seems that you need CMake 2.8.11 to use target_include_directories (http://www.cmake.org/files/v2.8/CMakeChangeLog-2.8.11). I couldn't find out which version is being used on the build machine, but it has to be an earlier version.

There are two solutions to this: The first is to increase the version requirement to 2.8.11 and upgrade CMake on the build machine. The other one is to check if CMake version is equal or greater than 2.8.11 before using target_include_directories.

I implemented the latter in the latest commit and it builds on the build bot and works as expected together with my project.

[cdunn2001]

Thanks!