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.

tests/sex-programs/c99.sex

@@ -0,0 +1,28 @@
+(compilation "-- -std=c99 -pedantic-errors")
+(input)
+(output "c99: 42")
+(return 0)
+
+;;; The closure environment is part of the ABI, so its union is declared
+;;; in every translation unit whether or not one is used. That put
+;;; whatever it was written with into every program: `max_align_t' named
+;;; the alignment in one word, and made C11 the floor for a program with
+;;; no closure in it at all.
+;;;
+;;; The widest built-ins say the same thing -- a union is aligned for the
+;;; strictest of its members -- and say it in C99.
+;;;
+;;; Closures themselves still want C11 for the `_Static_assert' that
+;;; checks the captures fit, so this program keeps clear of them.
+
+(include stdio.h)
+
+(struct point ((x int) (y int)))
+
+(fn area ((p (struct point))) int
+  (return (* (. p x) (. p y))))
+
+(pub fn main () int
+  (var p (struct point) #((struct point) : 6 7))
+  (printf "c99: %d\n" (area p))
+  (return 0))