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

build/cmake/policies.cmake

@@ -58,3 +58,8 @@ if(POLICY CMP0072)
     # FindOpenGL prefers GLVND by default when available.
     cmake_policy(SET CMP0072 NEW)
 endif()
+
+if(POLICY CMP0092)
+    # MSVC warning flags are not in CMAKE_<LANG>_FLAGS by default
+    cmake_policy(SET CMP0092 NEW)
+endif()