Fix compilation with GCC > 4.6 and Clang.

This compatibility stuff is killing me.

src/Magnum/Implementation/FramebufferState.cpp

@@ -34,9 +34,10 @@
 namespace Magnum { namespace Implementation {
 
 #ifndef CORRADE_GCC46_COMPATIBILITY
-constexpr
-#endif
+constexpr const Range2Di FramebufferState::DisengagedViewport;
+#else
 const Range2Di FramebufferState::DisengagedViewport{{}, {-1, -1}};
+#endif
 
 FramebufferState::FramebufferState(Context& context, std::vector<std::string>& extensions): readBinding{0}, drawBinding{0}, renderbufferBinding{0}, maxDrawBuffers{0}, maxColorAttachments{0}, maxRenderbufferSize{0}, maxSamples{0},
     #ifndef MAGNUM_TARGET_GLES

src/Magnum/Implementation/FramebufferState.h

@@ -43,11 +43,11 @@
 namespace Magnum { namespace Implementation {
 
 struct FramebufferState {
-    /* Also, GCC 4.6 doesn't have non-static member initializers */
     #ifndef CORRADE_GCC46_COMPATIBILITY
-    constexpr
-    #endif
+    constexpr static const Range2Di DisengagedViewport{{}, {-1, -1}};
+    #else
     static const Range2Di DisengagedViewport;
+    #endif
 
     explicit FramebufferState(Context& context, std::vector<std::string>& extensions);