build: do not treat compiler warnings as errors for unsafe builds with msvc

this is the same behaviour as with gcc and clang

Signed-off-by: Carlos Gomes Martinho <carlos.gomes_martinho@siemens.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1e3098c..c5fd562 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -162,7 +162,7 @@
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} --warn_about_c_style_casts --warnings_are_errors -Ohz")
 endif(CMAKE_COMPILER_IS_IAR)
 
-if(CMAKE_COMPILER_IS_MSVC)
+if(CMAKE_COMPILER_IS_MSVC AND NOT UNSAFE_BUILD)
     # Strictest warnings, and treat as errors
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /W3")
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /WX")