winbuild: build with warning level 4

This is consistent with 7bc6456, which
changed the warning level from 3 to 4 for the Visual Studio project
files. Also define WIN32_LEAN_AND_MEAN consistently, which avoids
unnecessarily including stuff that emits a lot of level 4 warnings when
using older Windows SDK versions.

Closes #1667

winbuild/MakefileBuild.vc

@@ -56,11 +56,11 @@ CC = cl.exe
 !IF "$(VC)"=="6"
 CC_NODEBUG  = $(CC) /O2 /DNDEBUG
 CC_DEBUG    = $(CC) /Od /Gm /Zi /D_DEBUG /GZ
-CFLAGS     = /I. /I../lib /I../include /nologo /W3 /GX /DWIN32 /YX /FD /c /DBUILDING_LIBCURL
+CFLAGS     = /I. /I../lib /I../include /nologo /W4 /GX /DWIN32 /YX /FD /c /DBUILDING_LIBCURL /DWIN32_LEAN_AND_MEAN
 !ELSE
 CC_NODEBUG  = $(CC) /O2 /DNDEBUG
-CC_DEBUG    = $(CC) /Od /D_DEBUG /RTC1 /Z7 /LDd /W3
-CFLAGS      = /I. /I ../lib /I../include /nologo /W3 /EHsc /DWIN32 /FD /c /DBUILDING_LIBCURL /DWIN32_LEAN_AND_MEAN
+CC_DEBUG    = $(CC) /Od /D_DEBUG /RTC1 /Z7 /LDd
+CFLAGS      = /I. /I ../lib /I../include /nologo /W4 /EHsc /DWIN32 /FD /c /DBUILDING_LIBCURL /DWIN32_LEAN_AND_MEAN
 !ENDIF
 
 LFLAGS     = /nologo /machine:$(MACHINE)
@@ -93,7 +93,7 @@ PDB_NAME_DLL_DEBUG     = $(BASE_NAME_DEBUG).pdb
 
 # CURL Command section
 PROGRAM_NAME = curl.exe
-CURL_CFLAGS   =  /I../lib /I../include /nologo /W3 /EHsc /DWIN32 /FD /c
+CURL_CFLAGS   =  /I../lib /I../include /nologo /W4 /EHsc /DWIN32 /DWIN32_LEAN_AND_MEAN /FD /c
 CURL_LFLAGS   = /nologo /out:$(DIRDIST)\bin\$(PROGRAM_NAME) /subsystem:console /machine:$(MACHINE)
 CURL_RESFLAGS = /i../include