TVersionCheck is our odd way to assert that header files and libraries are in sync.

The idea is that when we include TObject.h our TU will get the current version of
ROOT. Then gVersionCheck goes in all .o files which contain TObject. This goes
in the libraries and if somebody wants to link against different header files
version, ROOT compalins.

This patch restricts the inclusion of TVersionCheck only through TObject which
was the original purpose of all this.

core/base/inc/TVersionCheck.h

@@ -21,6 +21,10 @@
 //                                                                      //
 //////////////////////////////////////////////////////////////////////////
 
+#ifndef ROOT_TObject
+#error "TVersionCheck.h must be included only by TObject. Please #include TObject.h"
+#endif
+
 #ifndef ROOT_RVersion
 #include "RVersion.h"
 #endif