array_length: Make usable as a constant expression

Do not use a statement expression in array_length, so that
array_length can be used at file scope and as a constant expression.
Instead, put the _Static_assert into a struct (as a declaration),
and nest this in the expression using a sizeof expression.

ChangeLog

@@ -1,3 +1,9 @@
+2019-02-07  Florian Weimer  <fweimer@redhat.com>
+
+	* include/array_length.h (array_length): Do not use a statement
+	expression and _Static_assert, so that array_length can be used at
+	file scope and as a constant expression.
+
 2019-02-07  Florian Weimer  <fweimer@redhat.com>
 
 	* support/xdlfcn.h (xdlmopen): Declare.

include/array_length.h

@@ -22,12 +22,12 @@
 /* array_length (VAR) is the number of elements in the array VAR.  VAR
    must evaluate to an array, not a pointer.  */
 #define array_length(var)                                               \
-  __extension__ ({                                                      \
+  (sizeof (var) / sizeof ((var)[0])                                     \
+   + 0 * sizeof (struct {                                               \
        _Static_assert (!__builtin_types_compatible_p                    \
                        (__typeof (var), __typeof (&(var)[0])),          \
                        "argument must be an array");                    \
-    sizeof (var) / sizeof ((var)[0]);                                   \
-  })
+   }))
 
 /* array_end (VAR) is a pointer one past the end of the array VAR.
    VAR must evaluate to an array, not a pointer.  */