CMake: Fix MSVC warning when using NMake

Fix warning: Command line warning D9025 : overriding '/W3' with '/W4'
Remove /W3, either via a regex or via new policy CMP0092 (CMake 3.15).

The policy has to be set before calling project(), otherwise the
CMAKE_<LANG>_FLAGS are already initialized.

See https://trac.wxwidgets.org/ticket/18438

CMakeLists.txt

@@ -23,10 +23,10 @@ if(APPLE AND NOT CMAKE_OSX_DEPLOYMENT_TARGET)
     set(CMAKE_OSX_DEPLOYMENT_TARGET 10.9 CACHE STRING "OS X Deployment Target")
 endif()
 
-project(wxWidgets)
-
 include(build/cmake/policies.cmake NO_POLICY_SCOPE)
 
+project(wxWidgets)
+
 # Initialize variables for quick access to wx root dir in sub dirs
 set(wxSOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR})
 set(wxBINARY_DIR ${CMAKE_BINARY_DIR})