CMake: Install to QT_PLUGIN_PATH when set in environment

Provides a workaround for issue #2.

CMakeLists.txt

@@ -30,24 +30,30 @@ target_link_libraries(qaseprite
             dio-lib
             render-lib)
 
-set(QT_INSTALL_PREFIX ${QT${QT_VERSION_MAJOR}_INSTALL_PREFIX})
-set(QT_INSTALL_PLUGINS ${QT${QT_VERSION_MAJOR}_INSTALL_PLUGINS})
-
-# If QT_INSTALL_PREFIX is empty, try to derive it from QT_DIR, cutting off the
-# last three components ("lib/cmake/Qt5") from the path
-if(NOT QT_INSTALL_PREFIX)
-  set(QT_INSTALL_PREFIX ${QT_DIR})
-  get_filename_component(QT_INSTALL_PREFIX ${QT_INSTALL_PREFIX} DIRECTORY)
-  get_filename_component(QT_INSTALL_PREFIX ${QT_INSTALL_PREFIX} DIRECTORY)
-  get_filename_component(QT_INSTALL_PREFIX ${QT_INSTALL_PREFIX} DIRECTORY)
-endif()
-
-# If QT_INSTALL_PLUGINS is empty, set it to "plugins"
-if(NOT QT_INSTALL_PLUGINS)
-  set(QT_INSTALL_PLUGINS "plugins")
+if(DEFINED ENV{QT_PLUGIN_PATH})
+  set(QT_PLUGIN_PATH $ENV{QT_PLUGIN_PATH})
+else()
+  set(QT_INSTALL_PREFIX ${QT${QT_VERSION_MAJOR}_INSTALL_PREFIX})
+  set(QT_INSTALL_PLUGINS ${QT${QT_VERSION_MAJOR}_INSTALL_PLUGINS})
+
+  # If QT_INSTALL_PREFIX is empty, try to derive it from QT_DIR, cutting off the
+  # last three components ("lib/cmake/Qt5") from the path
+  if(NOT QT_INSTALL_PREFIX)
+    set(QT_INSTALL_PREFIX ${QT_DIR})
+    get_filename_component(QT_INSTALL_PREFIX ${QT_INSTALL_PREFIX} DIRECTORY)
+    get_filename_component(QT_INSTALL_PREFIX ${QT_INSTALL_PREFIX} DIRECTORY)
+    get_filename_component(QT_INSTALL_PREFIX ${QT_INSTALL_PREFIX} DIRECTORY)
+  endif()
+
+  # If QT_INSTALL_PLUGINS is empty, set it to "plugins"
+  if(NOT QT_INSTALL_PLUGINS)
+    set(QT_INSTALL_PLUGINS "plugins")
+  endif()
+
+  set(QT_PLUGIN_PATH "${QT_INSTALL_PREFIX}/${QT_INSTALL_PLUGINS}")
 endif()
 
 install(
   TARGETS qaseprite
-  RUNTIME DESTINATION "${QT_INSTALL_PREFIX}/${QT_INSTALL_PLUGINS}/imageformats"
-  LIBRARY DESTINATION "${QT_INSTALL_PREFIX}/${QT_INSTALL_PLUGINS}/imageformats")
+  RUNTIME DESTINATION "${QT_PLUGIN_PATH}/imageformats"
+  LIBRARY DESTINATION "${QT_PLUGIN_PATH}/imageformats")