Port to GCC 2.95. First two problems reported by Michael Deutschmann in

<http://lists.gnu.org/archive/html/bug-bison/2006-09/msg00018.html>.

* src/parse-gram.y (symbol_declaration): Don't put statements
before declarations; it's not portable to C89.
* src/scan-code.l (handle_action_at): Likewise.

* src/scan-code.l: Always initialize braces_level; the old code
left it uninitialized and therefore had undefined behavior.

Don't attempt to redefine 'assert', since it runs afoul of
systems where standard headers (mistakenly) include <assert.h>.
Instead, define and use our own alternative, called 'aver'.
* src/reader.c: Don't include assert.h, since we no longer
use assert.
* src/scan-code.l: Likewise.
* src/system.h (assert): Remove, replacing with....
(aver): New function, taking a bool arg.  All uses changed.
* src/tables.c (pack_vector): Ensure that aver arg is bool,
not merely an integer.