cmake: Fix symbol detection on non-standard include paths

We're still in the FindModule scope and the global include directories
therefore very likely do not contain the required path.

I used list(INSERT ... 0) and list(REMOVE_AT ... 0) instead of the more
intuitive APPEND and REMOVE_ITEM because REMOVE_ITEM actually removes
all occurrences. That might affect the global state which is exactly not
what a find_package ought to do. If the minimal required CMake was
2.8.6+, one could have used CMakePushCheckState, but it isn't available
in 2.6.

CMakeModules/FindLibNETCONF2.cmake

@@ -71,9 +71,11 @@ else (LIBNETCONF2_LIBRARIES AND LIBNETCONF2_INCLUDE_DIRS)
 
   if (LIBNETCONF2_INCLUDE_DIR AND LIBNETCONF2_LIBRARY)
     set(LIBNETCONF2_FOUND TRUE)
-    # check compile flags
+    # check the configured options and make them available through cmake
+    list(INSERT CMAKE_REQUIRED_INCLUDES 0 "${LIBNETCONF2_INCLUDE_DIR}")
     check_symbol_exists("NC_ENABLED_SSH" "nc_client.h" LIBNETCONF2_ENABLED_SSH)
     check_symbol_exists("NC_ENABLED_TLS" "nc_client.h" LIBNETCONF2_ENABLED_TLS)
+    list(REMOVE_AT CMAKE_REQUIRED_INCLUDES 0)
   else (LIBNETCONF2_INCLUDE_DIR AND LIBNETCONF2_LIBRARY)
     set(LIBNETCONF2_FOUND FALSE)
   endif (LIBNETCONF2_INCLUDE_DIR AND LIBNETCONF2_LIBRARY)