Detect empty elements in dependency list

These errors are usually an extra trailing semi-colon, which is easy
to put in there for symmetry with the other lines. Give a warning on
this but proceed with the build as usual afterwards.

cmake/Modules/OpmPackage.cmake

@@ -199,9 +199,13 @@ macro (find_opm_package module deps header lib defs prog conf)
   set (_deps)
   foreach (_dep IN ITEMS ${deps})
 	separate_arguments (_args UNIX_COMMAND ${_dep})
-	find_and_append_package_to (${module} ${_args} ${${module}_QUIET})
-	list (GET _args 0 _name_only)
-	list (APPEND _deps ${_name_only})
+	if (_args)
+	  find_and_append_package_to (${module} ${_args} ${${module}_QUIET})
+	  list (GET _args 0 _name_only)
+	  list (APPEND _deps ${_name_only})
+	else (_args)
+	  message (WARNING "Empty dependency in find module for ${module} (check for trailing semi-colon)")
+	endif (_args)
   endforeach (_dep)
 
   # since find_and_append_package_to is a macro, this variable have