%define variables: support value changes in deprecation

* src/muscle-tab.c (define_directive): Be robust to "assignment"
containing '='.
(muscle_percent_variable_update): Upgrade "variant" to "api.value.type".
Support such upgrade patterns.
Adjust callers.

* data/bison.m4: Use api.value.type for variants.
* tests/c++.at: Adjust tests.

tests/c++.at

@@ -34,7 +34,7 @@ AT_DATA_GRAMMAR([list.yy],
 [[%debug
 %skeleton "lalr1.cc"
 %defines
-%define variant
+%define api.value.type variant
 ]m4_bpatsubst([$1], [\\n], [
 ])[
 
@@ -456,7 +456,7 @@ AT_DATA_GRAMMAR([[input.yy]],
 }
 
 %defines
-%define variant
+%define api.value.type variant
 %define parse.error verbose
 %define parse.trace
 %%