Convert run-time to build-time assertion

There's just one, in show_product().

Use new BUILD_ASSERT() there, because its contract is even simpler
than BUILD_ASSERT_ONE()'s.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>

include/misc.h

@@ -82,6 +82,13 @@ extern void (*oops_handler)(void);
 #define BUILD_ASSERT_ONE(cond) \
     (sizeof(char[1 - 2 * !(cond)]))
 
+/*
+ * Assert constant expression @cond.
+ * If @cond is zero, force a compilation error.
+ */
+#define BUILD_ASSERT(cond) \
+    ((void)BUILD_ASSERT_ONE(cond))
+
 void exit_nomem(void) ATTRIBUTE((noreturn));
 
 	/* return codes from command routines */