[libpng16] Allow an easy replacement of the default pre-built configuration

header with a custom header, via the make PNGLIBCONF_H_PREBUILT
macro.

scripts/makefile.gcc

diff --git a/scripts/makefile.gcc b/scripts/makefile.gcc
index 3e1b84f..8811a28 100644
--- a/scripts/makefile.gcc
+++ b/scripts/makefile.gcc
@@ -30,6 +30,10 @@
 # File extensions
 EXEEXT =
 
+# Pre-built configuration
+# See scripts/pnglibconf.mak for more options
+PNGLIBCONF_H_PREBUILT = scripts/pnglibconf.h.prebuilt
+
 # Variables
 OBJS =  png.o pngerror.o pngget.o pngmem.o pngpread.o \
 	pngread.o pngrio.o pngrtran.o pngrutil.o pngset.o \
@@ -38,8 +42,7 @@
 # Targets
 all: static
 
-# see scripts/pnglibconf.mak for more options
-pnglibconf.h: scripts/pnglibconf.h.prebuilt
+pnglibconf.h: $(PNGLIBCONF_H_PREBUILT)
 	$(CP) $< $@
 
 .c.o: