Platform: fix building of EglContext with Xcode.

The dreaded OBJECT libraries again. Reported the inconvenience here:
http://public.kitware.com/pipermail/cmake/2016-April/063178.html

src/Magnum/CMakeLists.txt

@@ -323,7 +323,7 @@ if(BUILD_TESTS)
     # Library with graceful assert for testing
     add_library(MagnumMathTestLib ${SHARED_OR_STATIC}
         $<TARGET_OBJECTS:MagnumMathObjects>
-        Math/dummy.cpp) # XCode workaround, see file comment for details
+        dummy.cpp) # XCode workaround, see file comment for details
     target_include_directories(MagnumMathTestLib PUBLIC $<TARGET_PROPERTY:Magnum,INTERFACE_INCLUDE_DIRECTORIES>)
     target_compile_definitions(MagnumMathTestLib PRIVATE "CORRADE_GRACEFUL_ASSERT")
     set_target_properties(MagnumMathTestLib PROPERTIES DEBUG_POSTFIX "-d")

src/Magnum/Platform/CMakeLists.txt

@@ -578,7 +578,9 @@ if(NEED_EGLCONTEXT OR WITH_EGLCONTEXT)
 
     # Also create proper static library, if requested
     if(WITH_EGLCONTEXT)
-        add_library(MagnumEglContext STATIC $<TARGET_OBJECTS:MagnumEglContextObjects>)
+        add_library(MagnumEglContext STATIC
+            $<TARGET_OBJECTS:MagnumEglContextObjects>
+            dummy.cpp) # XCode workaround, see file comment for details
         set_target_properties(MagnumEglContext PROPERTIES DEBUG_POSTFIX "-d")
         if(BUILD_STATIC_PIC)
             set_target_properties(MagnumEglContext PROPERTIES POSITION_INDEPENDENT_CODE ON)

src/Magnum/dummy.cpp

@@ -26,5 +26,5 @@
 /* XCode-generated project files have a limitation that you can't create a
    library target from OBJECT sources only and you have to provide at least one
    file. Otherwise the generated library gets replaced with self-pointing
-   symlink and the build then obviously fails with "File not found" error. That
-   affected the MagnumMathTestLib library and this file should fix it. */
+   symlink and the build then obviously fails with "File not found" error.
+   Related: http://public.kitware.com/pipermail/cmake/2016-April/063178.html */