Minimize yacc and lex output

Minimize yacc and lex output

This minimizes the "make" output from the yacc and lex rules,
following the same technique as the rest of the Makefile.

The lex rule had a special case to deal with the situation where flex
is not available.  I don't think this is needed, so I removed it.  If
flex is truly unavailable, the person building gdb can simply "touch"
the output file.

gdb/ChangeLog
2018-07-09  Tom Tromey  <tom@tromey.com>

	* Makefile.in (%.c: %.y): Use ECHO_YACC.
	(%.c: %.l): Use ECHO_LEX.  Just fail if flex not available.
	* silent-rules.mk (ECHO_YACC, ECHO_LEX): New variables.