Check if the target doc exists to add the dependency

Signed-off-by: ahcorde <[email protected]>

CMakeLists.txt

@@ -121,5 +121,8 @@ ign_create_docs(
     "${IGNITION-MATH_DOXYGEN_TAGFILE} = ${IGNITION-MATH_API_URL}"
 )
 
-# wait to build the doc to the autogenerated code
-add_dependencies(doc ignition-msgs6)
+# Wait to build the doc for the autogenerated code
+# The TARGET will exist if doxygen is installed
+if(TARGET doc)
+  add_dependencies(doc ignition-msgs${PROJECT_VERSION_MAJOR})
+endif()