Fix POSIX zlib/png building when system libs not available.

This also allows to remove code from zconf upstream that I always had to place there.

configure

@@ -610,43 +610,48 @@ else
 fi
 
 #
-# Check for ZLib
+# Check for zlib and libpng
 #
-echocheck "zlib"
 if test "$_build_zip" = yes ; then
     _zlib=no
     $_pkg_config --exists zlib && _zlib=yes
-
-    if test "$_zlib" = yes ; then
-        echo "`$_pkg_config --modversion zlib`"
-    else
-        echo "disabled"
-        _build_png=no
-        _build_zip=no
-    fi
 else
-    echo "disabled"
     _build_png=no
     _build_zip=no
 fi
 
-#
-# Check for libpng
-#
-echocheck "libpng"
 if test "$_build_png" = yes ; then
     _libpng=no
     $_pkg_config --exists libpng && _libpng=yes
 
-    if test "$_libpng" = yes ; then
-        echo "`$_pkg_config --modversion libpng`"
+    # The built-in libpng is tied to the built-in zlib's version, so if
+    # either library is missing, both built-in versions are used
+    if test "$_zlib" != yes || test "$_libpng" != yes ; then
+        _zlib=no
+        _libpng=no
+    fi
+fi
+
+echocheck "zlib"
+if test "$_build_zip" = yes ; then
+    if test "$_zlib" = yes ; then
+        echo "`$_pkg_config --modversion zlib`"
     else
-        echo "disabled"
-        _build_png=no
+        echo "built-in"
+    fi
+else
+    echo "disabled"
+fi
+
+echocheck "libpng"
+if test "$_build_png" = yes ; then
+    if test "$_libpng" = yes ; then
+        echo "`$_pkg_config --modversion libpng`"
+    else
+        echo "built-in"
     fi
 else
     echo "disabled"
-    _build_png=no
 fi
 
 #
@@ -804,12 +809,12 @@ LD=$CXX
 
 case $_host_os in
     unix)
-      DEFINES="$DEFINES -DBSPF_UNIX"
+      DEFINES="$DEFINES -DBSPF_UNIX -DHAVE_UNISTD_H"
       MODULES="$MODULES $SRC_OS/unix"
       INCLUDES="$INCLUDES -I$SRC_OS/unix"
       ;;
     darwin)
-      DEFINES="$DEFINES -DBSPF_UNIX -DMACOS_KEYS"
+      DEFINES="$DEFINES -DBSPF_UNIX -DMACOS_KEYS -DHAVE_UNISTD_H"
       MODULES="$MODULES $SRC_OS/unix"
       INCLUDES="$INCLUDES -I$SRC_OS/unix"
       if test "$have_clang" == yes; then

src/lib/zlib/zconf.h

@@ -8,13 +8,6 @@
 #ifndef ZCONF_H
 #define ZCONF_H
 
-/* Added by SA: 2018-12-18 */
-#if defined(BSPF_UNIX) || defined(BSPF_MACOS)
-#  define HAVE_UNISTD_H
-#endif
-#define HAVE_STDARG_H
-/***************************/
-
 /*
  * If you *really* need a unique prefix for all types and library functions,
  * compile with -DZ_PREFIX. The "standard" zlib should be compiled without it.

src/os/macos/stella.xcodeproj/project.pbxproj

@@ -3980,6 +3980,7 @@
 					SOUND_SUPPORT,
 					WINDOWED_SUPPORT,
 					BSPF_MACOS,
+					HAVE_UNISTD_H,
 					MAC_OSX,
 					"PNG_ARM_NEON_OPT=0",
 				);
@@ -4048,6 +4049,7 @@
 					SOUND_SUPPORT,
 					WINDOWED_SUPPORT,
 					BSPF_MACOS,
+					HAVE_UNISTD_H,
 					MAC_OSX,
 					"PNG_ARM_NEON_OPT=0",
 				);