Support externally installed cJSON.

Using an external cJSON installed under /usr/include was not possible before
as the cJSON headers within opm were still used due to relative paths.
With this commit move the copied cJSON source to external/cjson and
thus prevent them to be found if an externally installed cJSON is there.

opm/json/CMakeLists.txt

@@ -1,8 +1,9 @@
 set( json_source JsonObject.cpp )
 set( json_headers JsonObject.hpp )
 if (NOT HAVE_CJSON)
-    list(APPEND json_source cjson/cJSON.c)
-    set( CJSON_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR} )
+    list(APPEND json_source ${PROJECT_SOURCE_DIR}/external/cjson/cJSON.c)
+    set( CJSON_INCLUDE_DIR ${PROJECT_SOURCE_DIR}/external )
+    include_directories(${CJSON_INCLUDE_DIR})
     set(CJSON_LIBRARY "")
 endif()
 add_library(opmjson ${json_source})