diff -u tcl8.6.8/win/tcl.m4.old tcl8.6.8/win/tcl.m4
--- tcl8.6.8/win/tcl.m4.old	2018-04-30 11:33:58.300874726 +0200
+++ tcl8.6.8/win/tcl.m4	2018-04-30 11:03:56.403602865 +0200
@@ -806,31 +806,6 @@
 	LIBSUFFIX="\${DBGX}.lib"
 	LIBFLAGSUFFIX="\${DBGX}"
 
-	# This is a 2-stage check to make sure we have the 64-bit SDK
-	# We have to know where the SDK is installed.
-	# This magic is based on MS Platform SDK for Win2003 SP1 - hobbs
-	if test "$do64bit" != "no" ; then
-	    if test "x${MSSDK}x" = "xx" ; then
-		MSSDK="C:/Progra~1/Microsoft Platform SDK"
-	    fi
-	    MSSDK=`echo "$MSSDK" | sed -e 's!\\\!/!g'`
-	    PATH64=""
-	    case "$do64bit" in
-		amd64|x64|yes)
-		    MACHINE="AMD64" ; # assume AMD64 as default 64-bit build
-		    PATH64="${MSSDK}/Bin/Win64/x86/AMD64"
-		    ;;
-		ia64)
-		    MACHINE="IA64"
-		    PATH64="${MSSDK}/Bin/Win64"
-		    ;;
-	    esac
-	    if test ! -d "${PATH64}" ; then
-		AC_MSG_WARN([Could not find 64-bit $MACHINE SDK])
-	    fi
-	    AC_MSG_RESULT([   Using 64-bit $MACHINE mode])
-	fi
-
 	LIBS="netapi32.lib kernel32.lib user32.lib advapi32.lib userenv.lib ws2_32.lib"
 
 	case "x`echo \${VisualStudioVersion}`" in
@@ -841,35 +816,14 @@
 		    ;;
 	esac
 
-	if test "$do64bit" != "no" ; then
-	    # The space-based-path will work for the Makefile, but will
-	    # not work if AC_TRY_COMPILE is called.  TEA has the
-	    # TEA_PATH_NOSPACE to avoid this issue.
-	    # Check if _WIN64 is already recognized, and if so we don't
-	    # need to modify CC.
-	    AC_CHECK_DECL([_WIN64], [],
-			  [CC="\"${PATH64}/cl.exe\" -I\"${MSSDK}/Include\" \
-			 -I\"${MSSDK}/Include/crt\" \
-			 -I\"${MSSDK}/Include/crt/sys\""])
-	    RC="\"${MSSDK}/bin/rc.exe\""
-	    CFLAGS_DEBUG="-nologo -Zi -Od ${runtime}d"
-	    # Do not use -O2 for Win64 - this has proved buggy in code gen.
-	    CFLAGS_OPTIMIZE="-nologo -O1 ${runtime}"
-	    lflags="${lflags} -nologo -MACHINE:${MACHINE} -LIBPATH:\"${MSSDK}/Lib/${MACHINE}\""
-	    LINKBIN="\"${PATH64}/link.exe\""
-	    # Avoid 'unresolved external symbol __security_cookie' errors.
-	    # c.f. http://support.microsoft.com/?id=894573
-	    LIBS="$LIBS bufferoverflowU.lib"
-	else
-	    RC="rc"
-	    # -Od - no optimization
-	    # -WX - warnings as errors
-	    CFLAGS_DEBUG="-nologo -Z7 -Od -WX ${runtime}d"
-	    # -O2 - create fast code (/Og /Oi /Ot /Oy /Ob2 /Gs /GF /Gy)
-	    CFLAGS_OPTIMIZE="-nologo -O2 ${runtime}"
-	    lflags="${lflags} -nologo"
-	    LINKBIN="link"
-	fi
+	RC="rc"
+	# -Od - no optimization
+	# -WX - warnings as errors
+	CFLAGS_DEBUG="-nologo -Z7 -Od -WX ${runtime}d"
+	# -O2 - create fast code (/Og /Oi /Ot /Oy /Ob2 /Gs /GF /Gy)
+	CFLAGS_OPTIMIZE="-nologo -O2 ${runtime}"
+	lflags="${lflags} -nologo"
+	LINKBIN="link"
 
 	if test "$doWince" != "no" ; then
 	    # Set defaults for common evc4/PPC2003 setup

Diff finished.  Mon Apr 30 11:34:06 2018
