GCC 4.5 compatibility: better workaround for defaulted destructors.

Previous solution was breaking "constexprness" on GCC > 4.5, thus we
need to do it two different ways.

src/Platform/AbstractXApplication.h

@@ -289,7 +289,11 @@ class AbstractXApplication::InputEvent {
     protected:
         constexpr InputEvent(Modifiers modifiers): _modifiers(modifiers), _accepted(false) {}
 
+        #ifndef CORRADE_GCC45_COMPATIBILITY
+        ~InputEvent() = default;
+        #else
         ~InputEvent();
+        #endif
     #endif
 
     private:
@@ -297,7 +301,9 @@ class AbstractXApplication::InputEvent {
         bool _accepted;
 };
 
+#ifdef CORRADE_GCC45_COMPATIBILITY
 AbstractXApplication::InputEvent::~InputEvent() = default;
+#endif
 
 CORRADE_ENUMSET_OPERATORS(AbstractXApplication::InputEvent::Modifiers)
 

src/Platform/GlutApplication.h

@@ -366,13 +366,19 @@ class GlutApplication::InputEvent {
     protected:
         constexpr InputEvent(): _accepted(false) {}
 
+        #ifndef CORRADE_GCC45_COMPATIBILITY
+        ~InputEvent() = default;
+        #else
         ~InputEvent();
+        #endif
 
     private:
         bool _accepted;
 };
 
+#ifdef CORRADE_GCC45_COMPATIBILITY
 GlutApplication::InputEvent::~InputEvent() = default;
+#endif
 
 /**
 @brief Key event

src/Platform/NaClApplication.h

@@ -410,7 +410,11 @@ class NaClApplication::InputEvent {
     protected:
         constexpr InputEvent(Modifiers modifiers): _accepted(false), _modifiers(modifiers) {}
 
+        #ifndef CORRADE_GCC45_COMPATIBILITY
         ~InputEvent() = default;
+        #else
+        ~InputEvent();
+        #endif
     #endif
 
     private:
@@ -418,6 +422,10 @@ class NaClApplication::InputEvent {
         const Modifiers _modifiers;
 };
 
+#ifdef CORRADE_GCC45_COMPATIBILITY
+NaClApplication::InputEvent::~InputEvent() = default;
+#endif
+
 /**
 @brief Key event
 

src/Platform/Sdl2Application.h

@@ -349,14 +349,20 @@ class Sdl2Application::InputEvent {
     protected:
         constexpr explicit InputEvent(): _accepted(false) {}
 
+        #ifndef CORRADE_GCC45_COMPATIBILITY
+        ~InputEvent() = default;
+        #else
         ~InputEvent();
+        #endif
     #endif
 
     private:
         bool _accepted;
 };
 
+#ifdef CORRADE_GCC45_COMPATIBILITY
 Sdl2Application::InputEvent::~InputEvent() = default;
+#endif
 
 /**
 @brief Key event