Quiet a warning when CONNEXTDDS_DIR or NDDSHOME is not found. (#158)

Failing to find those is an acceptable situation; in that
case, we will end up just not building rmw_connextdds.
Quiet the warning to only a STATUS message so it doesn't
cause builds to go yellow.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>

rti_connext_dds_cmake_module/cmake/rti_build_helper.cmake

@@ -377,7 +377,7 @@ function(rti_load_connextddsdir)
             file(TO_CMAKE_PATH "$ENV{NDDSHOME}" connextdds_dir)
         endif()
         if("${connextdds_dir}" STREQUAL "")
-            message(WARNING "no CONNEXTDDS_DIR nor NDDSHOME specified")
+            message(STATUS "no CONNEXTDDS_DIR nor NDDSHOME specified")
         endif()
         set(CONNEXTDDS_DIR "${connextdds_dir}")
     endif()