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.

0 files changed, 0 insertions, 0 deletions