declare the closure environment in c99

`max_align_t' is C11 and the union goes into every unit, so a program
with no closure in it stopped building under -std=c99. `unify' also
bound a rigid variable one way round only.

Makefile

@@ -99,7 +99,7 @@ sextest:
 SEX_TEST_PROGRAMS = hello-world lists comments unicode serialize features \
                     feature-flags lambdas compound-literals closures fixpoint \
                     wildcards inference type-shapes unnamed-params \
-                    closure-signatures
+                    closure-signatures c99
 
 # Multi-module linking is checked end to end; see tests/modules/Makefile.
 check-modules: sexc