Silence deprecated warnings in internals of deprecated build.

Together with recent Corrade changes is Magnum compiling without any
noise again (i.e. just one line of output from Ninja).

src/Magnum/Framebuffer.cpp

@@ -221,7 +221,14 @@ Framebuffer& Framebuffer::attachTextureLayer(Framebuffer::BufferAttachment attac
 
 #ifdef MAGNUM_BUILD_DEPRECATED
 Framebuffer& Framebuffer::attachTexture2D(BufferAttachment attachment, Texture2D& texture, Int mipLevel) {
+    #ifdef __GNUC__
+    #pragma GCC diagnostic push
+    #pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+    #endif
     (this->*Context::current()->state().framebuffer->texture2DImplementation)(attachment, GLenum(texture.target()), texture.id(), mipLevel);
+    #ifdef __GNUC__
+    #pragma GCC diagnostic pop
+    #endif
     return *this;
 }
 #endif

src/Magnum/Test/DebugGLTest.cpp

@@ -111,7 +111,14 @@ void DebugGLTest::insertMessageFallback() {
 
 #ifdef MAGNUM_BUILD_DEPRECATED
 void DebugGLTest::deprecated() {
+    #ifdef __GNUC__
+    #pragma GCC diagnostic push
+    #pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+    #endif
     DebugMarker::mark("hello");
+    #ifdef __GNUC__
+    #pragma GCC diagnostic pop
+    #endif
 
     MAGNUM_VERIFY_NO_ERROR();
 }