modules: actually detect the MAGNUM_PLUGINS_DIR.

It now defaults to magnum/ subdirectory of dir where main Magnum library
was found. It was *annoying* to change /usr/local/lib/magnum to
/usr/lib/magnum _every time_.

doc/cmake.dox

@@ -44,10 +44,11 @@ variables:
 -   `MAGNUM_FOUND` -- Whether the library was found
 -   `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_DIR` -- Base directory with plugins, defaults to `magnum/`
+    subdirectory of dir where Magnum library was found. 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

modules/FindMagnum.cmake

@@ -7,9 +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
+#  MAGNUM_PLUGINS_DIR           - Base directory with plugins, defaults to
+#   `magnum/` subdirectory of dir where Magnum library was found. 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
@@ -404,7 +405,9 @@ if(MAGNUM_BUILD_DEPRECATED)
     set(MAGNUM_PLUGINS_INCLUDE_DIR ${MAGNUM_INCLUDE_DIR}/MagnumPlugins)
 endif()
 
-set(MAGNUM_PLUGINS_DIR ${MAGNUM_PLUGINS_INSTALL_DIR}
+# Get base plugin directory from main library location
+get_filename_component(_MAGNUM_LIBRARY_PATH ${MAGNUM_LIBRARY} PATH)
+set(MAGNUM_PLUGINS_DIR ${_MAGNUM_LIBRARY_PATH}/magnum
     CACHE PATH "Base directory where to look for Magnum plugins")
 
 # Plugin directories