Use qt_add_plugin

This changes the plugin's extension on macOS from .so to .dylib, which
will hopefully cause it to be automatically shipped by macdeployqt.

Closes #8

CMakeLists.txt

@@ -280,9 +280,15 @@ add_subdirectory(aseprite/src/dio       EXCLUDE_FROM_ALL)
 # qaseprite specific setup
 # ------------------------
 
-add_library(qaseprite MODULE
-  qaseprite.cpp
-  qaseprite.json
+if(QT_VERSION_MAJOR EQUAL 5)
+    include(cmake/Qt5Plugin.cmake)
+endif()
+
+qt_add_plugin(qaseprite SHARED CLASS_NAME AsepriteImagePlugin)
+
+target_sources(qaseprite
+    PRIVATE qaseprite.cpp
+            qaseprite.json
 )
 
 set_target_properties(qaseprite PROPERTIES AUTOMOC ON)

cmake/Qt5Plugin.cmake

@@ -0,0 +1,93 @@
+# This file contains the minimal code to compile a shared Qt plugin with CMake.
+# It's based on qt6_add_plugin from Qt6 with all code not related to shared
+# plugins (module libraries) removed. Internal functions have the prefix
+# _qt_wrap instead of _qt.
+
+function(_qt_wrap_internal_apply_shared_win_prefix_and_suffix target)
+    if(WIN32 AND NOT MSVC)
+            set_property(TARGET "${target}" PROPERTY IMPORT_SUFFIX ".a")
+            set_property(TARGET "${target}" PROPERTY PREFIX "")
+            set_property(TARGET "${target}" PROPERTY IMPORT_PREFIX "lib")
+    endif()
+endfunction()
+
+function(_qt_wrap_internal_set_up_static_runtime_library target)
+    if(QT_FEATURE_static_runtime)
+        if(MSVC)
+            set_property(TARGET ${target} PROPERTY
+                MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")
+        elseif(MINGW)
+            get_target_property(target_type ${target} TYPE)
+            if(target_type STREQUAL "EXECUTABLE")
+                set(link_option PRIVATE)
+            else()
+                set(link_option INTERFACE)
+            endif()
+            if(CLANG)
+                target_link_options(${target} ${link_option} "LINKER:-Bstatic")
+            else()
+                target_link_options(${target} ${link_option} "-static")
+            endif()
+        endif()
+    endif()
+endfunction()
+
+function(qt5_add_plugin target)
+    cmake_parse_arguments(PARSE_ARGV 1 arg "SHARED" "" "")
+
+    if(NOT arg_SHARED)
+        message(FATAL_ERROR "Only shared plugins can be built")
+    endif()
+
+    add_library(${target} MODULE ${arg_UNPARSED_ARGUMENTS})
+    _qt_wrap_internal_set_up_static_runtime_library(${target})
+    _qt_wrap_internal_apply_shared_win_prefix_and_suffix("${target}")
+
+    if(arg_MODULE AND APPLE)
+        # CMake defaults to using .so extensions for loadable modules, aka plugins,
+        # but Qt plugins are actually suffixed with .dylib.
+        set_property(TARGET "${target}" PROPERTY SUFFIX ".dylib")
+    endif()
+
+    if(ANDROID)
+        set_property(TARGET "${target}" PROPERTY SUFFIX "_${CMAKE_ANDROID_ARCH_ABI}.so")
+    endif()
+
+    set_property(TARGET ${target} PROPERTY _qt_expects_finalization TRUE)
+
+    set(output_name ${target})
+    if (arg_OUTPUT_NAME)
+        set(output_name ${arg_OUTPUT_NAME})
+    endif()
+    set_property(TARGET "${target}" PROPERTY OUTPUT_NAME "${output_name}")
+
+    if (ANDROID)
+        set_target_properties(${target}
+            PROPERTIES
+            LIBRARY_OUTPUT_NAME "plugins_${arg_PLUGIN_TYPE}_${output_name}"
+        )
+    endif()
+
+    # Derive the class name from the target name if it's not explicitly specified.
+    if (NOT arg_CLASS_NAME)
+        set(plugin_class_name "${target}")
+    else()
+        set(plugin_class_name "${arg_CLASS_NAME}")
+    endif()
+
+    set_target_properties(${target} PROPERTIES QT_PLUGIN_CLASS_NAME "${plugin_class_name}")
+
+    target_compile_definitions(${target} PRIVATE
+        QT_PLUGIN
+        QT_DEPRECATED_WARNINGS
+    )
+
+    if(NOT TARGET qt_wrap_internal_plugins)
+        add_custom_target(qt_wrap_internal_plugins)
+    endif()
+    add_dependencies(qt_wrap_internal_plugins ${target})
+endfunction()
+
+function(qt_add_plugin)
+    qt5_add_plugin(${ARGV})
+endfunction()