bug#25706: 26.0.50; Slow C file fontification

[Mattias Engdegård]

> Actually, it's the underscores!

Found it. Suggested fix attached.

It can be improved: at least one pair of regexp group brackets can be removed, 
but I didn't dare doing so because I wasn't sure if it would throw some group 
numbers off by one.

Alan, please, let's work together and remove unnecessary capture groups from 
the regexps! Even XEmacs regexps support non-capturing brackets, \(?:...\), and 
they save time, regexp stack space, and reduce the hassle of computing the 
'regexp depth' everywhere.

cc-underscores.diff
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