cmake: Enable the libpng framework build by default on Apple systems

The PNG_FRAMEWORK option used to be off by default. It was possible to
turn it on, regardless of the underlying operating system, but doing so
outside of an Apple OS broke the libpng build.

PNG_FRAMEWORK is now on by default, conditionally defined on Apple
systems only, and it is ignored (without breaking the build) elsewhere.

Other minor changes have also been applied.

Author: ctruta

CMakeLists.txt

@@ -68,14 +68,17 @@ set(PNG_DEBUG_POSTFIX "d"
 set(DFA_XTRA ""
     CACHE FILEPATH "File containing extra configuration settings")
 
-# Allow the users to configure the following build options.
-option(PNG_SHARED "Build libpng as a shared lib" ON)
-option(PNG_STATIC "Build libpng as a static lib" ON)
-option(PNG_FRAMEWORK "Build libpng as a Mac OS X framework" OFF)
+# Allow the users to switch on/off various library build types.
+option(PNG_SHARED "Build libpng as a shared library" ON)
+option(PNG_STATIC "Build libpng as a static library" ON)
+if(APPLE)
+  option(PNG_FRAMEWORK "Build libpng as a framework bundle" ON)
+endif()
+
+# Allow the users to switch on/off the auxiliary build and test artifacts.
+# NOTE: These artifacts are NOT part of libpng proper, and are subject to change at any time.
 option(PNG_TESTS "Build the libpng tests" ON)
 option(PNG_TOOLS "Build the libpng tools" ON)
-option(PNG_DEBUG "Enable debug output" OFF)
-option(PNG_HARDWARE_OPTIMIZATIONS "Enable hardware optimizations" ON)
 
 # Maintain backwards compatibility with the deprecated option PNG_EXECUTABLES.
 option(PNG_EXECUTABLES "[Deprecated; please use PNG_TOOLS]" ON)
@@ -88,6 +91,10 @@ if(NOT PNG_EXECUTABLES)
   endif()
 endif()
 
+# Allow the users to configure various compilation options.
+option(PNG_DEBUG "Enable debug output" OFF)
+option(PNG_HARDWARE_OPTIMIZATIONS "Enable hardware optimizations" ON)
+
 # Allow the users to specify a location of zlib.
 # Useful if zlib is being built alongside this as a sub-project.
 option(PNG_BUILD_ZLIB "Custom zlib location, else find_package is used" OFF)
@@ -659,7 +666,11 @@ if(PNG_SHARED)
                             LINK_FLAGS "-Wl,-M -Wl,'${CMAKE_CURRENT_BINARY_DIR}/libpng.vers'")
     endif()
   endif()
-  if(WIN32)
+  if(APPLE)
+    # Avoid CMake's implicit compile definition "png_shared_EXPORTS".
+    set_target_properties(png_shared PROPERTIES DEFINE_SYMBOL "")
+  elseif(WIN32)
+    # Use the explicit compile definition "PNG_BUILD_DLL" for Windows DLLs.
     set_target_properties(png_shared PROPERTIES DEFINE_SYMBOL PNG_BUILD_DLL)
   endif()
   target_include_directories(png_shared
@@ -685,7 +696,7 @@ if(PNG_STATIC)
   target_link_libraries(png_static PUBLIC ZLIB::ZLIB ${M_LIBRARY})
 endif()
 
-if(PNG_FRAMEWORK)
+if(PNG_FRAMEWORK AND APPLE)
   add_library(png_framework SHARED ${libpng_sources})
   add_dependencies(png_framework png_genfiles)
   list(APPEND PNG_LIBRARY_TARGETS png_framework)
@@ -697,7 +708,10 @@ if(PNG_FRAMEWORK)
                         MACOSX_FRAMEWORK_IDENTIFIER "org.libpng.libpng"
                         XCODE_ATTRIBUTE_INSTALL_PATH "@rpath"
                         PUBLIC_HEADER "${libpng_public_hdrs}"
-                        OUTPUT_NAME "png")
+                        OUTPUT_NAME "png"
+                        DEBUG_POSTFIX "${PNG_DEBUG_POSTFIX}")
+  # Avoid CMake's implicit compile definition "-Dpng_framework_EXPORTS".
+  set_target_properties(png_framework PROPERTIES DEFINE_SYMBOL "")
   target_include_directories(png_framework
                              PUBLIC
                                $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}/libpng${PNGLIB_ABI_VERSION}>