Treat %error-verbose as %define error_verbose.

Treat %error-verbose as %define error_verbose.

This allows to pass -Derror_verbose on the command line.  Better yet, it
allows to pass -Derror_verbose=$(ERROR_VERBOSE), with ERROR_VERBOSE being
defined as false or true.

	* data/bison.m4 (b4_percent_define_if_define): Instead of relying
	on b4_percent_define_ifdef, for does not check the defined value,
	but only whether the symbol is defined, rely on
	b4_percent_define_flag_if, so that a value of "false" is processed
	as a false.
	If not defined, define the flag to "false".
	(b4_error_verbose_if): New.
	* data/glr.c, data/lalr1.cc, data/yacc.c: Use it instead of
	b4_error_verbose_flag.
	* src/getargs.h, src/getargs.c (error_verbose_flag): Remove.
	* src/output.c (prepare): Don't output it.
	* src/parse-gram.y (%error-verbose): Treat as %define error_verbose.