modules: link SDL to Metal framework on iOS.

Also do it only when SDL is static -- dynamic builds don't need any of
that.

doc/changelog.dox

@@ -150,6 +150,9 @@ See also:
     and @ref Magnum/Platform/GlfwApplication.h (see [mosra/magnum#389](https://github.com/mosra/magnum/pull/389))
 -   `FindSDL2.cmake` got updated to link to all dependencies when using a
     statically built SDL on Windows
+-   `FindSDL2.cmake` now links to Metal in addition to other frameworks on iOS
+    and is updated to link to these only if SDL is statically built (see
+    [mosra/magnum#410](https://github.com/mosra/magnum/issues/410))
 -   `FindMagnum.cmake` now properly recognizes an optional dependency between
     @ref DebugTools and @ref Trade on GL-less builds
 -   Various compiler warning fixes (see [mosra/magnum#406](https://github.com/mosra/magnum/pull/406))

modules/FindSDL2.cmake

@@ -117,8 +117,8 @@ if(CORRADE_TARGET_WINDOWS)
         PATH_SUFFIXES ${_SDL2_RUNTIME_PATH_SUFFIX} ${_SDL2_LIBRARY_PATH_SUFFIX})
 endif()
 
-# iOS dependencies
-if(CORRADE_TARGET_IOS)
+# (Static) iOS dependencies
+if(CORRADE_TARGET_IOS AND SDL2_LIBRARY MATCHES ".*libSDL2.a$")
     set(_SDL2_FRAMEWORKS
         AudioToolbox
         AVFoundation
@@ -126,6 +126,7 @@ if(CORRADE_TARGET_IOS)
         CoreMotion
         Foundation
         GameController
+        Metal # needed since 2.0.8
         QuartzCore
         UIKit)
     set(_SDL2_FRAMEWORK_LIBRARIES )
@@ -173,8 +174,8 @@ if(NOT TARGET SDL2::SDL2)
                 INTERFACE_LINK_LIBRARIES Threads::Threads ${CMAKE_DL_LIBS})
         endif()
 
-        # Link frameworks on iOS
-        if(CORRADE_TARGET_IOS)
+        # Link frameworks on iOS if we have a static SDL
+        if(CORRADE_TARGET_IOS AND SDL2_LIBRARY MATCHES ".*libSDL2.a$")
             set_property(TARGET SDL2::SDL2 APPEND PROPERTY
                 INTERFACE_LINK_LIBRARIES ${_SDL2_FRAMEWORK_LIBRARIES})
         endif()