Windows: Fix inconsistent behavior on changes to case of path to cmake

In commit v3.4.0-rc1~333^2 (Merge branch 'upstream-kwsys' into
update-kwsys, 2015-07-15) we brought in upstream KWSys commit 86a24794
(SystemTools: Fix GetActualCaseForPath drive letter case handling,
2015-07-09).  This caused our path processing to convert drive letters
to upper-case and exposed an existing bug in our implementation of
CMP0017.

Policy CMP0017 is responsible for ensuring that modules included from a
builtin module only load other builtin modules and cannot be overridden
by a file in `CMAKE_MODULE_PATH`.  If there is a case difference in the
drive letter (or other path components) then the path to the including
module may not match our builtin module directory in a simple string
comparison.  This means builtin modules may not be recognized as such,
and they may not reliably include their builtin dependencies.  For
example, if a project provides a `Platform/Windows` module in
`CMAKE_MODULE_PATH` it can break inclusion of our builtin
`Platform/Windows` module, leading to strange behavior.

Fix this by comparing the path to the including module to our builtin
module directory using a function that is aware of case-insensitivity of
paths on Windows.

Fixes: #16648, #16622

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