Support CMake 3.6

Newer CMake versions have the following problems with our current source tree:

• The <FLAGS> rule variable has been split into <INCLUDES> and <FLAGS> in CMake 3.4.0 – see https://github.com/Kitware/CMake/commit/c736de7b284ecc93bac48106e88417e0e6c92ad6
• CMake has a new policy CMP0058, and by default complains about files specified as DEPENDS that are listed neither as OUTPUTS nor BYPRODUCTS of an existing command – see https://cmake.org/cmake/help/v3.3/policy/CMP0058.html

To fix the first we simply must add <INCLUDES> everywhere that <FLAGS> is found.
The second problem concerns two files for us: reactos.dff.dyn and reactos.inf. The first is generated using file(WRITE) by CMake so does not need a rule, the second is extremely painful to test as-is, so setting the policy to OLD for now seems safest.