diff -ur orig/gdal-3.1.3/frmts/gtiff/libtiff/makefile.vc ../frmts/gtiff/libtiff/makefile.vc
--- orig/gdal-3.1.3/frmts/gtiff/libtiff/makefile.vc	2020-09-01 10:36:30.000000000 +0200
+++ ../frmts/gtiff/libtiff/makefile.vc	2020-10-11 16:12:10.142034500 +0200
@@ -56,14 +56,14 @@
 !IFDEF JPEG_EXTERNAL_LIB
 JPEG_FLAGS =	-I$(JPEGDIR) -DJPEG_SUPPORT  -DOJPEG_SUPPORT
 !ELSE
-JPEG_FLAGS =	-I..\..\jpeg\libjpeg -DJPEG_SUPPORT -DOJPEG_SUPPORT
+EXTRAFLAGS =	-I..\..\jpeg\libjpeg -DJPEG_SUPPORT -DOJPEG_SUPPORT $(EXTRAFLAGS)
 !ENDIF
 !ENDIF
 
 !IFDEF ZLIB_EXTERNAL_LIB
 ZLIB_FLAGS = $(ZLIB_INC)
 !ELSE
-ZLIB_FLAGS = -I..\..\zlib
+EXTRA_FLAGS = -I..\..\zlib $(EXTRA_FLAGS)
 !ENDIF
 
 !IFDEF JPEG12_SUPPORTED
diff -ur orig/gdal-3.1.3/frmts/gtiff/makefile.vc ../frmts/gtiff/makefile.vc
--- orig/gdal-3.1.3/frmts/gtiff/makefile.vc	2020-09-01 10:36:30.000000000 +0200
+++ ../frmts/gtiff/makefile.vc	2020-10-11 16:12:10.140034100 +0200
@@ -39,7 +39,7 @@
 !IFDEF GEOTIFF_INC
 SUB_GEOTIFF_TARGET =	
 !ELSE
-GEOTIFF_INC   =	-Ilibgeotiff -DINTERNAL_LIBGEOTIFF
+EXTRAFLAGS	=	-Ilibgeotiff -DINTERNAL_LIBGEOTIFF $(EXTRAFLAGS)
 SUB_GEOTIFF_TARGET =	geotiff
 !ENDIF
 
@@ -47,7 +47,7 @@
 !IFDEF JPEG_EXTERNAL_LIB
 JPEG_FLAGS =	-I..\jpeg -I$(JPEGDIR) -DHAVE_LIBJPEG
 !ELSE
-JPEG_FLAGS =	-I..\jpeg -I..\jpeg\libjpeg -DHAVE_LIBJPEG
+EXTRAFLAGS =	-I..\jpeg -I..\jpeg\libjpeg -DHAVE_LIBJPEG $(EXTRAFLAGS)
 !ENDIF
 !ENDIF
 
