Fix CMAKE_DISABLE_SOURCE_CHANGES recognition of top of build tree

The change in commit v3.11.0-rc1~480^2 (UseJava: add_jar OUTPUT_DIR
option used only for jar generation, 2017-10-12) added code of the form
`file(MAKE_DIRECTORY ${CMAKE_BINARY_DIR})`.  This exposed an existing
bug in `CMAKE_DISABLE_SOURCE_CHANGES` in which it does not recognize
that the top of the build tree itself is in the build tree.  Fix that
now.

Fixes: #17933

Source/cmMakefile.cxx

@@ -2304,7 +2304,8 @@ bool cmMakefile::CanIWriteThisFile(std::string const& fileName) const
   }
 
   return !cmSystemTools::IsSubDirectory(fileName, this->GetHomeDirectory()) ||
-    cmSystemTools::IsSubDirectory(fileName, this->GetHomeOutputDirectory());
+    cmSystemTools::IsSubDirectory(fileName, this->GetHomeOutputDirectory()) ||
+    cmSystemTools::SameFile(fileName, this->GetHomeOutputDirectory());
 }
 
 const char* cmMakefile::GetRequiredDefinition(const std::string& name) const