maint: use static_assert

* gnulib.modules: Add assert-h, for static_assert.
* src/common.h, src/list.c, src/misc.c:
Prefer static_assert to #if + #error.  This doesn’t fix any bugs; it’s
just that in general it’s better to avoid the preprocessor.

gnulib.modules

@@ -22,6 +22,7 @@ areadlinkat-with-size
 argmatch
 argp
 argp-version-etc
+assert-h
 attribute
 backupfile
 c-ctype