warnings: avoid warnings from clang

Fix the following warning

  parse-gram.c:2078:14: error: equality comparison with extraneous parentheses
                        [-Werror,-Wparentheses-equality]
    if (((yyn) == (-91)))
         ~~~~~~^~~~~~~~
  parse-gram.c:2078:14: note: remove extraneous parentheses around the
                        comparison to silence this warning
    if (((yyn) == (-91)))
        ~      ^       ~
  parse-gram.c:2078:14: note: use '=' to turn this equality comparison into
                        an assignment
    if (((yyn) == (-91)))
               ^~
               =
  1 error generated.

and the following one:

  input.cc:740:1: error: function declared 'noreturn' should not return
                         [-Werror,-Winvalid-noreturn]
  static void yyMemoryExhausted (yyGLRStack* yystackp)
    __attribute__ ((__noreturn__));
  static void
  yyMemoryExhausted (yyGLRStack* yystackp)
  {
    YYLONGJMP (yystackp->yyexception_buffer, 2);
  }
  ^
  1 warning and 1 error generated.

This is Apple clang version 3.1 (tags/Apple/clang-318.0.61).

* data/c.m4 (b4_table_value_equals): Use (!!(A == B)) instead of (A == B)
to avoid this warning.
Any reasonable compiler should generate the same code.
* src/uniqstr.h (UNIQSTR_EQ): Likewise.
* data/glr.c (LONGJMP): abort after longjmp to pacify clang.

data/c.m4

@@ -184,7 +184,7 @@ m4_define([b4_table_value_equals],
 [m4_if(m4_eval($3 < m4_indir([b4_]$1[_min])
                || m4_indir([b4_]$1[_max]) < $3), [1],
        [[YYID (0)]],
-       [[((]$2[) == (]$3[))]])])
+       [(!!(($2) == ($3)))])])
 
 
 ## ---------##

data/glr.c

@@ -290,7 +290,8 @@ b4_percent_code_get[]dnl
 # include <setjmp.h>
 # define YYJMP_BUF jmp_buf
 # define YYSETJMP(Env) setjmp (Env)
-# define YYLONGJMP(Env, Val) longjmp (Env, Val)
+// Pacify clang.
+# define YYLONGJMP(Env, Val) (longjmp (Env, Val), YYASSERT (0))
 #endif
 
 /*-----------------.