The approach taken by commit 8704525f (Reject directory names containing
'=', 2011-01-14) was perhaps too heavy-handed for avoiding the obscure
cases when '=' in the path fails due to limitations of Make syntax.
Only two CMake tests:

  LinkDirectory
  OutOfSource

fail when the path contains '=' and they cover obscure cases.  Instead
of rejecting such paths outright just warn when the problem may occur.