Windows and Apple machines have predominantly used case-insensitive
filesystems so our file(INSTALL) command uses case-insensitive pattern
matching.  It is implemented by converting the pattern and file path to
lower case before matching.  The FILES_MATCHING option is implemented by
excluding a path that does not match any pattern unless it is a
directory that must be searched recursively.  However, the test that an
excluded path is a directory is executed on the lower-case path and
therefore fails on mixed-case input paths on case-sensitive filesystems.
Fix the file(INSTALL) implementation to use the lower-case path only for
pattern matching and preserve the original path for tests against the
real filesystem.