[cmake] use /W4 with visual studio

CMakeLists.txt

@@ -116,16 +116,19 @@ if(NOT WIN32)
 add_compile_definitions(_FORTIFY_SOURCE=2)
 add_compile_options(-Wformat -Werror=format-security)
 endif()
+if(MSVC)
+add_compile_options(/W4)
+else()
 add_compile_options(-Wall -Wextra -W -Wshadow -Wvla)
 # -ffast-math dies on NaNs we draw from libav (by -ffinite-math-only)
 add_compile_options(-fno-signed-zeros -fno-trapping-math -fassociative-math)
 add_compile_options(-fno-math-errno -freciprocal-math -funsafe-math-optimizations)
 add_compile_options(-fexceptions -fstrict-aliasing)
-
 if(${USE_ASAN})
 add_compile_options(-fsanitize=address)
 add_link_options(-fsanitize=address)
 endif()
+endif()
 
 # don't use REQUIRED with subsequent find_package() operations; we use
 # feature_summary + set_package_properties to fail in one fell swoop.