cmake 2.8.12 if Qt is present warns about policy CMP0020

From bruen...@google.com on April 08, 2014 16:55:42

-- Found Qt 5: DrGUI will be built CMake Warning (dev) at CMakeLists.txt:1084 (export): Policy CMP0020 is not set: Automatically link Qt executables to qtmain target on Windows. Run "cmake --help-policy CMP0020" for policy details. Use the cmake_policy command to set the policy and suppress this warning. Call Stack (most recent call first): ext/drgui/CMakeLists.txt:85 (DR_export_target) This warning is for project developers. Use -Wno-dev to suppress it.

Xref a similar cmake policy warning involving interface imports: https://code.google.com/p/drmemory/issues/detail?id=1481 For now let's set the policy to OLD and resolve both issues with interface imports later once more people are on 2.8.12 and we can use the new features.

Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1418