modules: don't fail w/ confusing error if plugin include dir is not found.

Introduced by accident when implementing automagic import of static
plugins.

modules/FindMagnum.cmake

@@ -765,8 +765,10 @@ foreach(_component ${Magnum_FIND_COMPONENTS})
             mark_as_advanced(_MAGNUM_${_COMPONENT}_INCLUDE_DIR)
         endif()
 
-        # Automatic import of static plugins on CMake >= 3.1
+        # Automatic import of static plugins on CMake >= 3.1. Skip in case the
-        if(_component MATCHES ${_MAGNUM_PLUGIN_COMPONENTS} AND NOT CMAKE_VERSION VERSION_LESS 3.1)
+        # include dir was not found -- that'll fail later with a proper
+        # message.
+        if(_component MATCHES ${_MAGNUM_PLUGIN_COMPONENTS} AND NOT CMAKE_VERSION VERSION_LESS 3.1 AND _MAGNUM_${_COMPONENT}_INCLUDE_DIR)
             # Automatic import of static plugins
             file(READ ${_MAGNUM_${_COMPONENT}_INCLUDE_DIR}/configure.h _magnum${_component}Configure)
             string(FIND "${_magnum${_component}Configure}" "#define MAGNUM_${_COMPONENT}_BUILD_STATIC" _magnum${_component}_BUILD_STATIC)