get default mac build working with libpng + system libz.

without enable_all=on, png was getting mixed signals as it wasn't defining the prefix (as there was a system libz), but then not actually adding the system libz to the link line.  result was unresolved zlib symbols.  this tell png to turn off libz building, so it'll use the system libz, if our parent config has building zlib turned off.

Author: brlcad

png/CMakeLists.txt

@@ -36,6 +36,7 @@ if (ENABLE_PNG)
     $<$<BOOL:${ZLIB_TARGET}>:-DZ_PREFIX=ON>
     $<$<BOOL:${ZLIB_TARGET}>:-DZ_PREFIX_STR=${Z_PREFIX_STR}>
     -DZLIB_ROOT=$<$<BOOL:${ZLIB_TARGET}>:${CMAKE_BUNDLE_INSTALL_PREFIX}>
+    $<$<BOOL:${ZLIB_TARGET}>:-DPNG_BUILD_ZLIB=OFF>
     -DPNG_LIB_NAME=${PNG_LIB_NAME}
     -DPNG_NO_DEBUG_POSTFIX=ON
     -DPNG_PREFIX=brl_

png/png.patch

@@ -138,7 +138,7 @@ diff -Naur libpng/CMakeLists.txt PNG_BLD/CMakeLists.txt
  if(POLICY CMP0074)
    # Allow find_package() to use the ZLIB_ROOT variable, if available.
    cmake_policy(SET CMP0074 NEW)
-@@ -47,8 +50,10 @@
+@@ -48,8 +51,10 @@
  # Previously, we used to set CMAKE_DEBUG_POSTFIX globally. That variable should
  # not be cached, however, because doing so would affect all projects processed
  # after libpng, in unexpected and undesirable ways.
@@ -151,15 +151,15 @@ diff -Naur libpng/CMakeLists.txt PNG_BLD/CMakeLists.txt
 
  # Allow the users to import their own extra configuration settings.
  set(DFA_XTRA ""
-@@ -97,6 +102,7 @@
+@@ -98,6 +103,7 @@
  option(PNG_BUILD_ZLIB "Custom zlib location, else find_package is used" OFF)
  if(NOT PNG_BUILD_ZLIB)
    find_package(ZLIB REQUIRED)
 +  include_directories(${ZLIB_INCLUDE_DIRS})
  elseif(POLICY CMP0074)
    if("x${ZLIB_ROOT}" STREQUAL "x")
      message(DEPRECATION
-@@ -489,10 +495,9 @@
+@@ -490,10 +496,9 @@
 
    if(PNG_PREFIX)
      set(PNGLIBCONF_H_EXTRA_DEPENDS
@@ -172,7 +172,7 @@ diff -Naur libpng/CMakeLists.txt PNG_BLD/CMakeLists.txt
    endif()
 
    generate_out(INPUT "${CMAKE_CURRENT_BINARY_DIR}/pnglibconf.c"
-@@ -676,15 +681,15 @@
+@@ -677,15 +682,15 @@
    # We are on a Unix or Unix-like toolchain like the GNU toolchain on Windows.
    # Library file names are expected to have an implicit prefix such as "lib".
    # Let CMake prepend and append its usual prefixes and suffixes by default.
@@ -192,7 +192,7 @@ diff -Naur libpng/CMakeLists.txt PNG_BLD/CMakeLists.txt
  endif()
 
  if(PNG_SHARED)
-@@ -1067,16 +1072,16 @@
+@@ -1068,16 +1073,16 @@
    if(PNG_SHARED)
      # Create a symlink for libpng.dll.a => libpng16.dll.a on Cygwin
      if(NOT WIN32 OR CYGWIN OR MINGW)