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.

semen.scm

@@ -693,13 +693,19 @@
 
 (define +closure-env-bytes+ 16)
 
-;;; +closure-env-bytes+ for maximum capacity, max-align-t for
-;;; effectiveness, hence union
+;;; +closure-env-bytes+ for maximum capacity, and an alignment wide
+;;; enough for anything that fits in them, hence union. `max_align_t'
+;;; would say that in one word, but it is C11 and the target is C99, so
+;;; the widest built-ins say it instead: a union is aligned for the
+;;; strictest of its members.
 (define +closure-env-type+ 'ƛenv)
 
 (define (closure-env-declaration)
-  `(union ,+closure-env-type+ ((align max-align-t)
-                               (bytes (¤ char ,+closure-env-bytes+)))))
+  `(union ,+closure-env-type+ ((bytes (¤ char ,+closure-env-bytes+))
+                               (align-integer (long long))
+                               (align-real (long double))
+                               (align-pointer (* void))
+                               (align-code (fn ((* void)) void)))))
 
 (define +closure-structs+ (make-hash-table))
 (define +closure-forwards+ (make-hash-table))