Use MSVC warning level 4.

asio/src/Makefile.msc

diff --git a/asio/src/Makefile.msc b/asio/src/Makefile.msc
index 73ef8e0..9a0f2b3 100644
--- a/asio/src/Makefile.msc
+++ b/asio/src/Makefile.msc
@@ -5,7 +5,7 @@
 COMMON_CXXFLAGS = -nologo -EHac -GR -I. -I../include -I$(BOOSTDIR)
 
 !ifdef WARNINGS
-WARNINGS_CXXFLAGS = -W3 -Wp64
+WARNINGS_CXXFLAGS = -W4 -Wp64
 !endif
 
 !ifdef STATICRTL