libcpp: add function to check XID properties

This commit adds a new function intended for checking the XID properties
of a possibly unicode character, as well as the accompanying enum
describing the possible properties.

libcpp/ChangeLog:

	* charset.cc (cpp_check_xid_property): New.
	* include/cpplib.h
	(cpp_check_xid_property): New.
	(enum cpp_xid_property): New.

Signed-off-by: Raiki Tamura <tamaron1203@gmail.com>