FPII-2034: DO NOT MERGE Update libpng to 1.6.20

DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I2fc2106965c789a54f04412e9c63a0901b0c1e71
(cherry picked from commit 0b39f202aaedd3029e4270d2af7d58ba6634f9cd)
diff --git a/scripts/makefile.bc32 b/scripts/makefile.bc32
index c2a9122..bdbc4cb 100644
--- a/scripts/makefile.bc32
+++ b/scripts/makefile.bc32
@@ -11,10 +11,11 @@
 ## Where zlib.h, zconf.h and zlib.lib are
 ZLIB_DIR=..\zlib
 
-## Compiler, linker and lib stuff
+## Compiler, linker, librarian and other tools
 CC=bcc32
 LD=bcc32
 LIB=tlib
+CP=copy
 
 # -3 = 386, -4 = 486, -5 = Pentium etc.
 !ifndef TARGET_CPU
@@ -49,6 +50,12 @@
 # -M  generate map file
 LDFLAGS=-L$(ZLIB_DIR) -M $(LDEBUG)
 
+# Pre-built configuration
+# See scripts\pnglibconf.mak for more options
+!ifndef PNGLIBCONF_H_PREBUILT
+PNGLIBCONF_H_PREBUILT = scripts\pnglibconf.h.prebuilt
+!endif
+
 ## Variables
 OBJS = \
 	png.obj \
@@ -111,10 +118,8 @@
 
 ## Minor Targets
 
-# see scripts\pnglibconf.mak for how to make this file
-# with different options
-pnglibconf.h: scripts\pnglibconf.h.prebuilt
-	copy scripts\pnglibconf.h.prebuilt $@
+pnglibconf.h: $(PNGLIBCONF_H_PREBUILT)
+	$(CP) $(PNGLIBCONF_H_PREBUILT) $@
 
 png.obj: png.c png.h pngconf.h pnglibconf.h pngpriv.h pngstruct.h pnginfo.h pngdebug.h
 pngerror.obj: pngerror.c png.h pngconf.h pnglibconf.h pngpriv.h pngstruct.h pnginfo.h pngdebug.h