2014-05-31  Ed Smith-Rowland  <3dw4rd@verizon.net>

	Implement SD-6: SG10 Feature Test Recommendations
	* directives.c: Support __has_include__ builtin.
	* expr.c (parse_has_include): New function to parse __has_include__
	builtin; (eval_token()): Use it.
	* files.c (_cpp_has_header()): New funtion to look for header;
	(open_file_failed()): Not an error to not find a header file for
	__has_include__.
	* identifiers.c (_cpp_init_hashtable()): Add entry for __has_include__.
	* internal.h (lexer_state, spec_nodes): Add in__has_include__.
	* pch.c (cpp_read_state): Lookup __has_include__.
	* traditional.c (enum ls, _cpp_scan_out_logical_line()): Walk through
	__has_include__ statements.