CMake: make MAGNUM_PLUGINS[_DEBUG|_RELEASE|]_DIR a STRING cache entry.

The PATH entry is always converted to absolute, making it impossible to
specify a relative path (e.g. relative to executable location).

CMakeLists.txt

@@ -303,13 +303,16 @@ else()
     set(MAGNUM_PLUGINS_DIR_INIT ${MAGNUM_PLUGINS_RELEASE_DIR_INIT})
 endif()
 
-# Make these paths configurable from outside
+# Make these paths configurable from outside. This is *not* PATH, because CMake
+# always converts the path to an absolute location internally, making it
+# impossible to specify relative paths there. Sorry in advance for not having
+# the dir selection button in CMake GUI.
 set(MAGNUM_PLUGINS_DEBUG_DIR ${MAGNUM_PLUGINS_DEBUG_DIR_INIT}
-    CACHE PATH "Base directory where to look for Magnum plugins for debug builds")
+    CACHE STRING "Base directory where to look for Magnum plugins for debug builds")
 set(MAGNUM_PLUGINS_RELEASE_DIR ${MAGNUM_PLUGINS_RELEASE_DIR_INIT}
-    CACHE PATH "Base directory where to look for Magnum plugins for release builds")
+    CACHE STRING "Base directory where to look for Magnum plugins for release builds")
 set(MAGNUM_PLUGINS_DIR ${MAGNUM_PLUGINS_DIR_INIT}
-    CACHE PATH "Base directory where to look for Magnum plugins")
+    CACHE STRING "Base directory where to look for Magnum plugins")
 
 add_subdirectory(modules)
 add_subdirectory(src)

modules/FindMagnum.cmake

@@ -794,13 +794,16 @@ set(MAGNUM_PLUGINS_AUDIOIMPORTER_RELEASE_LIBRARY_INSTALL_DIR ${MAGNUM_PLUGINS_RE
 set(MAGNUM_INCLUDE_INSTALL_DIR ${CMAKE_INSTALL_PREFIX}/include/Magnum)
 set(MAGNUM_PLUGINS_INCLUDE_INSTALL_DIR ${CMAKE_INSTALL_PREFIX}/include/MagnumPlugins)
 
-# Get base plugin directory from main library location
+# Get base plugin directory from main library location. This is *not* PATH,
+# because CMake always converts the path to an absolute location internally,
+# making it impossible to specify relative paths there. Sorry in advance for
+# not having the dir selection button in CMake GUI.
 set(MAGNUM_PLUGINS_DEBUG_DIR ${_MAGNUM_PLUGINS_DIR_PREFIX}/magnum-d
-    CACHE PATH "Base directory where to look for Magnum plugins for debug builds")
+    CACHE STRING "Base directory where to look for Magnum plugins for debug builds")
 set(MAGNUM_PLUGINS_RELEASE_DIR ${_MAGNUM_PLUGINS_DIR_PREFIX}/magnum
-    CACHE PATH "Base directory where to look for Magnum plugins for release builds")
+    CACHE STRING "Base directory where to look for Magnum plugins for release builds")
 set(MAGNUM_PLUGINS_DIR ${_MAGNUM_PLUGINS_DIR_PREFIX}/magnum${_MAGNUM_PLUGINS_DIR_SUFFIX}
-    CACHE PATH "Base directory where to look for Magnum plugins")
+    CACHE STRING "Base directory where to look for Magnum plugins")
 
 # Plugin directories
 set(MAGNUM_PLUGINS_FONT_DIR ${MAGNUM_PLUGINS_DIR}/fonts)