CMake: proper absolute MAGNUM_PLUGINS_DIR variables.

Broken since 05ed476446 (June 2017). Oh
well.

CMakeLists.txt

@@ -306,16 +306,16 @@ set(MAGNUM_PLUGINS_AUDIOIMPORTER_RELEASE_LIBRARY_INSTALL_DIR ${MAGNUM_PLUGINS_RE
 # Plugin dirs based on wheter we are in debug or release build, needed by some
 # command-line tools
 if(CORRADE_TARGET_WINDOWS)
-    set(MAGNUM_PLUGINS_DEBUG_DIR_INIT ${MAGNUM_PLUGINS_DEBUG_BINARY_INSTALL_DIR})
-    set(MAGNUM_PLUGINS_RELEASE_DIR_INIT ${MAGNUM_PLUGINS_RELEASE_BINARY_INSTALL_DIR})
+    set(MAGNUM_PLUGINS_DEBUG_DIR_INIT ${CMAKE_INSTALL_PREFIX}/${MAGNUM_PLUGINS_DEBUG_BINARY_INSTALL_DIR})
+    set(MAGNUM_PLUGINS_RELEASE_DIR_INIT ${CMAKE_INSTALL_PREFIX}/${MAGNUM_PLUGINS_RELEASE_BINARY_INSTALL_DIR})
 else()
-    set(MAGNUM_PLUGINS_DEBUG_DIR_INIT ${MAGNUM_PLUGINS_DEBUG_LIBRARY_INSTALL_DIR})
-    set(MAGNUM_PLUGINS_RELEASE_DIR_INIT ${MAGNUM_PLUGINS_RELEASE_LIBRARY_INSTALL_DIR})
+    set(MAGNUM_PLUGINS_DEBUG_DIR_INIT ${CMAKE_INSTALL_PREFIX}/${MAGNUM_PLUGINS_DEBUG_LIBRARY_INSTALL_DIR})
+    set(MAGNUM_PLUGINS_RELEASE_DIR_INIT ${CMAKE_INSTALL_PREFIX}/${MAGNUM_PLUGINS_RELEASE_LIBRARY_INSTALL_DIR})
 endif()
 if(CMAKE_BUILD_TYPE STREQUAL "Debug")
-    set(MAGNUM_PLUGINS_DIR_INIT ${MAGNUM_PLUGINS_DEBUG_DIR_INIT})
+    set(MAGNUM_PLUGINS_DIR_INIT ${CMAKE_INSTALL_PREFIX}/${MAGNUM_PLUGINS_DEBUG_DIR_INIT})
 else()
-    set(MAGNUM_PLUGINS_DIR_INIT ${MAGNUM_PLUGINS_RELEASE_DIR_INIT})
+    set(MAGNUM_PLUGINS_DIR_INIT ${CMAKE_INSTALL_PREFIX}/${MAGNUM_PLUGINS_RELEASE_DIR_INIT})
 endif()
 
 # Make these paths configurable from outside. This is *not* PATH, because CMake

doc/changelog.dox

@@ -101,6 +101,8 @@ See also:
 -   @ref Attribute::DataType::HalfFloat was not available on WebGL 2 by mistake
 -   A wrong code path for @ref Framebuffer::checkStatus() was selected on WebGL
     1 by mistake
+-   Fixed `MAGNUM_PLUGINS_DIR` variables to contain proper absolute location by
+    default again.
 
 @subsection changelog-latest-deprecated Deprecated APIs