diagnostics: treat obsolete %define variable names as obsolete directives

Instead of

  warning: deprecated %define variable name: 'namespace', use 'api.namespace' [-Wdeprecated]

display (in -fno-caret mode):

  warning: deprecated directive: '%define namespace foo', use '%define api.namespace foo' [-Wdeprecated]

and (in -fcaret mode):

  warning: deprecated directive, use '%define api.namespace toto' [-Wdeprecated]
   %define namespace toto
           ^^^^^^^^^

This is in preparation of cases where not only the variable is
renamed, but the values are too:

  warning: deprecated directive: '%define variant', use '%define api.value.type variant' [-Wdeprecated]

* src/muscle-tab.c (define_directive): New.
(muscle_percent_variable_update): Take the value as argument, and use it
in the diagnostics.
Loop with a pointer instead of an index.
* tests/input.at (%define backward compatibility): Adjust.