GHS: Update toolset selection logic

-- Ensure that GHS_TOOLSET_ROOT is used as a path
   * Converts directory path slashes to CMake style
-- Use ComparePath() to properly check for path changes of build tool
   * Accounts for Windows file-system case insensitivity.
-- Don't print message "defaulting" messages (this causes CMake test failures)
-- Don't force update CMAKE_GENERATOR_TOOLSET back into cache when `-T`
   is not used on initial configure. This change avoids an unnessary
   error message when accidentally using `-T` in subsequent runs but the same
   tools are always used.
-- Use IssueMessage() for error messages.