modules: give MAGNUM_DEPLOY_PREFIX a saner default.

Otherwise, when used as-is in the install() command, CMake complains
that DESTINATION can't be empty.

modules/FindMagnum.cmake

@@ -11,8 +11,8 @@
 #
 #  Magnum_FOUND                 - Whether the base library was found
 #  MAGNUM_DEPLOY_PREFIX         - Prefix where to put final application
-#   executables, defaults to empty string. If a relative path is used, it's
-#   relative to :variable:`CMAKE_INSTALL_PREFIX`.
+#   executables, defaults to ``.``. If a relative path is used, it's relative
+#   to :variable:`CMAKE_INSTALL_PREFIX`.
 #  MAGNUM_PLUGINS_DEBUG_DIR     - Base directory with dynamic plugins for
 #   debug builds, defaults to magnum-d/ subdirectory of dir where Magnum
 #   library was found
@@ -794,7 +794,7 @@ if(_MAGNUM_CONTEXT_ALIAS AND NOT TARGET Magnum::Context)
 endif()
 
 # Installation and deploy dirs
-set(MAGNUM_DEPLOY_PREFIX ""
+set(MAGNUM_DEPLOY_PREFIX "."
     CACHE STRING "Prefix where to put final application executables")
 
 include(${CORRADE_LIB_SUFFIX_MODULE})