Make picojson a submodule

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "vendor/picojson"]
+	path = vendor/picojson
+	url = https://github.com/kazuho/picojson.git

CMakeLists.txt

@@ -11,6 +11,27 @@ if(NOT XROOTD_PLUGIN_VERSION)
   set(XROOTD_PLUGIN_VERSION ${XROOTD_PLUGIN_VERSION} CACHE INTERNAL "")
 endif()
 
+# Check out picojson submodule
+find_package(Git QUIET)
+if(GIT_FOUND AND EXISTS "${PROJECT_SOURCE_DIR}/.git")
+# Update submodules as needed
+    option(GIT_SUBMODULE "Check submodules during build" ON)
+    if(GIT_SUBMODULE)
+        message(STATUS "Submodule update")
+        execute_process(COMMAND ${GIT_EXECUTABLE} submodule update --init --recursive
+                        WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
+                        RESULT_VARIABLE GIT_SUBMOD_RESULT)
+        if(NOT GIT_SUBMOD_RESULT EQUAL "0")
+            message(FATAL_ERROR "git submodule update --init failed with ${GIT_SUBMOD_RESULT}, please checkout submodules")
+        endif()
+    endif()
+endif()
+
+if(NOT EXISTS "${PROJECT_SOURCE_DIR}/vendor/picojson/picojson.h")
+    message(FATAL_ERROR "The submodules were not downloaded! GIT_SUBMODULE was turned off or failed. Please update submodules and try again.")
+endif()
+
+
 macro(use_cxx11)
   if (CMAKE_VERSION VERSION_LESS "3.1")
     if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")