According to Mastering Visual Studio book the ExtensibilityGlobals (and ExtensibilityAddIns) section included for the benefit of add-in authors. ExtensibilityGlobals used to store global information about the solution. So it is clearly generated by(or for) some 3rd party tools.
The only discussion about SolutionGuid is here, which is generated by CMake and they advise that you should not touch this property.
Sometimes you can safely delete some properties from this section like discussed here.