Use MacPorts zlib and libpng.

https://github.com/XEphem/XEphem/issues/111

Fixes build failure on macOS 26.

https://github.com/XEphem/XEphem/issues/110
--- Makefile.orig	2025-10-26 23:15:26.000000000 -0500
+++ Makefile	2025-10-26 23:20:58.000000000 -0500
@@ -9,8 +9,8 @@
 # one executable, xephem.
 
 # These -I and -L flags point to the supporting XEphem libraries
-LIBINC = -I../../libastro -I../../libip -I../../liblilxml -I../../libjpegd -I../../libpng -I../../libz
-LIBLNK = -L../../libastro -L../../libip -L../../liblilxml -L../../libjpegd -L../../libpng -L../../libz
+LIBINC = -I../../libastro -I../../libip -I../../liblilxml -I../../libjpegd
+LIBLNK = -L../../libastro -L../../libip -L../../liblilxml -L../../libjpegd
 LIBLIB = -lastro -lip -llilxml -ljpegd -lpng -lz
 
 # MOTIFI is the directory containing the Xm directory of include files.
@@ -188,7 +188,7 @@
 	xephem.o	\
 	xmisc.o
 
-LIBRARIES := libip liblilxml libjpegd libpng libz
+LIBRARIES := libip liblilxml libjpegd
 
 all: $(LIBRARIES) xephem xephem.1