Fixed compilation with Clang's libc++.

src/Context.h

@@ -28,6 +28,7 @@
  * @brief Enum Magnum::Version, class Magnum::Context, Magnum::Extension, macro MAGNUM_ASSERT_VERSION_SUPPORTED(), MAGNUM_ASSERT_EXTENSION_SUPPORTED()
  */
 
+#include <cstdlib>
 #include <bitset>
 #include <vector>
 #include <Containers/EnumSet.h>

src/Platform/Implementation/GlxContextHandler.cpp

@@ -24,6 +24,7 @@
 
 #include "GlxContextHandler.h"
 
+#include <cstdlib>
 #include <GL/glxext.h>
 #include <Utility/Debug.h>