CheckIncludeFiles: Shorten check description message
Since commit 44d007b6 (CheckIncludeFiles: fix status output, 2012-02-01) check_include_files reports the list of files tested instead of the name of the variable storing the result. Some projects incrementally test and concatenate very long lists leading to long messages that do not provide much information. Users report confusion especially when the lines wrap. For lists of more than two files produce messages of the format Looing for N include files first.h, ..., last.h where N is the list length and "..." is literal. Leave the log file entries and cache entry description unchanged as they should have the full detail of the check performed.