Fix lookup of source names after conversion to their actual case (#15259)

Since commit v3.1.0-rc1~688^2~15 (cmTarget: Add a method to obtain list of
filenames for sources, 2014-03-17) we have code paths that lookup sources by
strings containing their own full path after normalization to the actual case
on disk.  This fails in the case that a cmSourceFile has already been created
with a different case in the filename.  The comparison of the directory works
because it is always normalized.  Only the comparison of the file name fails.

Fix this by using a case-insensitive comparison of source file names on
platforms that do not have case-sensitive filesystems.
Merge topic 'fix-source-case-matching' into next

84d124e8 Fix lookup of source names after conversion to their actual case (#15259)