Allow multi-byte character literals

[LIJI32]

Removed the optional length check when parsing character literals (e.g. 'IHDR'), making cppp able to process header files that make use of this commonly-used extension.

[BR903]

Unfortunately, this deletion is not enough by itself to correctly handle multicharacter literals. If such literals are permitted, then they can also appear in preprocessor expressions, so cppp needs to be able to correctly evaluate them as well. (For certain interpretations of "correctly" anyway, since their actual values are implementation-defined.)

(Also, please note that this code is actually out of date. The website https://www.muppetlabs.com/~breadbox/software/cppp.html always points to the current version, which is at https://git.sr.ht/~breadbox/cppp. My apologies for the confusion.)

[andreasabel]

@BR903 Maybe archive this repo and point to the current repo in the README.