Index: tcl-core-8.6.4.x/generic/tclMain.c
===================================================================
--- tcl-core-8.6.4.x/generic/tclMain.c	(revision 89060)
+++ tcl-core-8.6.4.x/generic/tclMain.c	(working copy)
@@ -77,15 +77,18 @@
 /*
  * Declarations for various library functions and variables (don't want to
  * include tclPort.h here, because people might copy this file out of the Tcl
- * source directory to make their own modified versions).
+ * source directory to make their own modified versions).
+ */
+
+#if defined _MSC_VER && _MSC_VER < 1900
+/* isatty is always defined on MSVC 14.0, but not necessarily as CRTIMPORT. */
+extern CRTIMPORT int	isatty(int fd);
+#endif
+
+/*
+ * The thread-local variables for this file's functions.
  */
 
-extern CRTIMPORT int	isatty(int fd);
-
-/*
- * The thread-local variables for this file's functions.
- */
-
 typedef struct {
     Tcl_Obj *path;		/* The filename of the script for *_Main()
 				 * routines to [source] as a startup script,
Index: tcl-core-8.6.4.x/win/makefile.vc
===================================================================
--- tcl-core-8.6.4.x/win/makefile.vc	(revision 89060)
+++ tcl-core-8.6.4.x/win/makefile.vc	(working copy)
@@ -71,7 +71,7 @@
 #		Sets where to install Tcl from the built binaries.
 #		C:\Progra~1\Tcl is assumed when not specified.
 #
-#	OPTS=loimpact,msvcrt,nothreads,pdbs,profile,static,staticpkg,symbols,thrdalloc,tclalloc,unchecked,none
+#	OPTS=loimpact,msvcrt,nothreads,pdbs,profile,static,staticpkg,symbols,thrdalloc,tclalloc,unchecked,ucrt,none
 #		Sets special options for the core.  The default is for none.
 #		Any combination of the above may be used (comma separated).
 #		'none' will over-ride everything to nothing.
@@ -102,6 +102,11 @@
 #		unchecked= Allows a symbols build to not use the debug
 #			   enabled runtime (msvcrt.dll not msvcrtd.dll
 #			   or libcmt.lib not libcmtd.lib).
+#		ucrt= Uses ucrt.lib and libvcruntime.lib, which
+#			   ensures Tcl will run on machines with only the subset
+#			   of the C runtime that is part of the operating system.
+#			   If omitted, builds with VC 14.0 or later will require
+#			   the full C runtime redistributable.
 #
 #	STATS=compdbg,memdbg,none
 #		Sets optional memory and bytecode compiler debugging code added
@@ -488,10 +493,16 @@
 cwarn = $(WARNINGS) -D _CRT_SECURE_NO_DEPRECATE -D _CRT_NONSTDC_NO_DEPRECATE
 cflags = -nologo -c $(COMPILERFLAGS) $(cwarn) -Fp$(TMP_DIR)^\
 
-!if $(MSVCRT)
+!if $(UCRT)
 !if $(DEBUG) && !$(UNCHECKED)
 crt = -MDd
 !else
+crt = -MT
+!endif
+!elseif $(MSVCRT)
+!if $(DEBUG) && !$(UNCHECKED)
+crt = -MDd
+!else
 crt = -MD
 !endif
 !else
@@ -530,6 +541,10 @@
 lflags	= $(lflags) -profile
 !endif
 
+!if $(UCRT) && !($(DEBUG) && !$(UNCHECKED))
+lflags	= $(lflags) -nodefaultlib:libucrt.lib
+!endif
+
 !if $(ALIGN98_HACK) && !$(STATIC_BUILD)
 ### Align sections for PE size savings.
 lflags	= $(lflags) -opt:nowin98
@@ -554,6 +569,9 @@
 baselibs   = $(baselibs) bufferoverflowU.lib
 !endif
 !endif
+!if $(UCRT) && !($(DEBUG) && !$(UNCHECKED))
+baselibs   = $(baselibs) ucrt.lib
+!endif
 
 #---------------------------------------------------------------------
 # TclTest flags
Index: tcl-core-8.6.4.x/win/rules.vc
===================================================================
--- tcl-core-8.6.4.x/win/rules.vc	(revision 89060)
+++ tcl-core-8.6.4.x/win/rules.vc	(working copy)
@@ -223,6 +223,7 @@
 TCL_USE_STATIC_PACKAGES	= 0
 USE_THREAD_ALLOC = 1
 UNCHECKED	= 0
+UCRT		= 0
 !else
 !if [nmakehlp -f $(OPTS) "static"]
 !message *** Doing static
@@ -302,6 +303,12 @@
 UNCHECKED = 0
 !endif
 !endif
+!if [nmakehlp -f $(OPTS) "ucrt"] && $(VCVERSION) >= 1900
+!message *** Doing UCRT
+UCRT = 1
+!else
+UCRT = 0
+!endif
 
 #----------------------------------------------------------
 # Figure-out how to name our intermediate and output directories.
Index: tk-8.6.4.x/win/makefile.vc
===================================================================
--- tk-8.6.4.x/win/makefile.vc	(revision 89060)
+++ tk-8.6.4.x/win/makefile.vc	(working copy)
@@ -76,7 +76,7 @@
 #		Sets where to install Tcl from the built binaries.
 #		C:\Progra~1\Tcl is assumed when not specified.
 #
-#	OPTS=loimpact,msvcrt,nothreads,noxp,pdbs,profile,square,static,staticpkg,symbols,unchecked,none
+#	OPTS=loimpact,msvcrt,nothreads,noxp,pdbs,profile,square,static,staticpkg,symbols,unchecked,ucrt,none
 #		Sets special options for the core.  The default is for none.
 #		Any combination of the above may be used (comma separated).
 #		'none' will over-ride everything to nothing.
@@ -105,6 +105,11 @@
 #		unchecked = Allows a symbols build to not use the debug
 #			   enabled runtime (msvcrt.dll not msvcrtd.dll
 #			   or libcmt.lib not libcmtd.lib).
+#		ucrt= Uses ucrt.lib and libvcruntime.lib, which
+#			   ensures Tcl will run on machines with only the subset
+#			   of the C runtime that is part of the operating system.
+#			   If omitted, builds with VC 14.0 or later will require
+#			   the full C runtime redistributable.
 #
 #	STATS=compdbg,memdbg,none
 #		Sets optional memory and bytecode compiler debugging code added
@@ -460,10 +465,16 @@
 cwarn = $(WARNINGS) -D _CRT_SECURE_NO_DEPRECATE -D _CRT_NONSTDC_NO_DEPRECATE
 cflags = -nologo -c $(COMPILERFLAGS) $(cwarn) -Fp$(TMP_DIR)^\
 
-!if $(MSVCRT)
+!if $(UCRT)
 !if $(DEBUG) && !$(UNCHECKED)
 crt = -MDd
 !else
+crt = -MT
+!endif
+!elseif $(MSVCRT)
+!if $(DEBUG) && !$(UNCHECKED)
+crt = -MDd
+!else
 crt = -MD
 !endif
 !else
@@ -500,6 +511,10 @@
 lflags	= $(lflags) -profile
 !endif
 
+!if $(UCRT) && !($(DEBUG) && !$(UNCHECKED))
+lflags	= $(lflags) -nodefaultlib:libucrt.lib
+!endif
+
 !if $(ALIGN98_HACK) && !$(STATIC_BUILD)
 ### Align sections for PE size savings.
 lflags	= $(lflags) -opt:nowin98
@@ -526,6 +541,11 @@
 baselibs   = $(baselibs) bufferoverflowU.lib
 !endif
 !endif
+
+!if $(UCRT) && !($(DEBUG) && !$(UNCHECKED))
+baselibs   = $(baselibs) ucrt.lib
+!endif
+
 guilibs	  = $(baselibs) gdi32.lib
 
 
Index: tk-8.6.4.x/win/rules.vc
===================================================================
--- tk-8.6.4.x/win/rules.vc	(revision 89060)
+++ tk-8.6.4.x/win/rules.vc	(working copy)
@@ -223,6 +223,7 @@
 TCL_USE_STATIC_PACKAGES	= 0
 USE_THREAD_ALLOC = 1
 UNCHECKED	= 0
+UCRT	= 0
 !else
 !if [nmakehlp -f $(OPTS) "static"]
 !message *** Doing static
@@ -302,6 +303,12 @@
 UNCHECKED = 0
 !endif
 !endif
+!if [nmakehlp -f $(OPTS) "ucrt"] && $(VCVERSION) >= 1900
+!message *** Doing UCRT
+UCRT = 1
+!else
+UCRT = 0
+!endif
 
 #----------------------------------------------------------
 # Figure-out how to name our intermediate and output directories.