yacc.c: initialize yylval and yylloc.

yacc.c: initialize yylval and yylloc.

When generating a pure push parser, the initialization of yylval and
yylloc may not be visible to the compiler.  With warnings enabled, GCC
4.3.6, 4.4.7, 4.5.4, and 4.6.3 report uninitialized uses of
yylval/yylloc.  Using local pragmas to disable these warnings is not
supported before 4.6, and 4.6 does not support it properly.  So
initialize yylval and yylloc at their definition.  Reported by Peter
Simons.  See
http://lists.gnu.org/archive/html/bison-patches/2012-10/msg00133.html

* data/c.m4 (b4_yyloc_default_define): New.
* data/yacc.c: Use it when locations are requested.
(YYLVAL_INITIALIZE): Replace by...
(YY_INITIAL_VALUE): this.
(yyparse): Initialize yylloc and yylval.
Therefore, remove the initialization of yylloc's field.
* data/glr.c: Likewise.