FindMagnum.cmake: allow user to override plugin directories.

Useful when deploying on Windows.

modules/FindMagnum.cmake

@@ -7,6 +7,10 @@
 #  MAGNUM_LIBRARIES             - Magnum library and dependent libraries
 #  MAGNUM_INCLUDE_DIRS          - Root include dir and include dirs of
 #   dependencies
+#  MAGNUM_PLUGINS_DIR           - Base directory with plugins. You can modify
+#   it (e.g. set it to `.` when deploying on Windows with plugins stored
+#   relatively to the executable), the following MAGNUM_PLUGINS_*_DIR
+#   variables depend on it.
 #  MAGNUM_PLUGINS_FONT_DIR      - Directory with font plugins
 #  MAGNUM_PLUGINS_FONTCONVERTER_DIR - Directory with font converter plugins
 #  MAGNUM_PLUGINS_IMAGECONVERTER_DIR - Directory with image converter plugins
@@ -341,15 +345,11 @@ mark_as_advanced(FORCE
     MAGNUM_INCLUDE_INSTALL_DIR
     MAGNUM_PLUGINS_INCLUDE_INSTALL_DIR)
 
+set(MAGNUM_PLUGINS_DIR ${MAGNUM_PLUGINS_INSTALL_DIR}
+    CACHE PATH "Base directory where to look for Magnum plugins")
+
 # Plugin directories
-if(NOT WIN32)
-    set(MAGNUM_PLUGINS_FONT_DIR ${MAGNUM_PLUGINS_INSTALL_DIR}/fonts)
-    set(MAGNUM_PLUGINS_FONTCONVERTER_DIR ${MAGNUM_PLUGINS_INSTALL_DIR}/fontconverters)
-    set(MAGNUM_PLUGINS_IMAGECONVERTER_DIR ${MAGNUM_PLUGINS_INSTALL_DIR}/imageconverters)
-    set(MAGNUM_PLUGINS_IMPORTER_DIR ${MAGNUM_PLUGINS_INSTALL_DIR}/importers)
-else()
-    set(MAGNUM_PLUGINS_FONT_DIR fonts)
-    set(MAGNUM_PLUGINS_FONTCONVERTER_DIR fontconverters)
-    set(MAGNUM_PLUGINS_IMAGECONVERTER_DIR imageconverters)
-    set(MAGNUM_PLUGINS_IMPORTER_DIR importers)
-endif()
+set(MAGNUM_PLUGINS_FONT_DIR ${MAGNUM_PLUGINS_DIR}/fonts)
+set(MAGNUM_PLUGINS_FONTCONVERTER_DIR ${MAGNUM_PLUGINS_DIR}/fontconverters)
+set(MAGNUM_PLUGINS_IMAGECONVERTER_DIR ${MAGNUM_PLUGINS_DIR}/imageconverters)
+set(MAGNUM_PLUGINS_IMPORTER_DIR ${MAGNUM_PLUGINS_DIR}/importers)