Don't define wxADJUST_MINSIZE if it had been already defined.

Don't define wxADJUST_MINSIZE if it had been already defined.

Defining wxADJUST_MINSIZE as 0 may be a simple way to make the old code to
compile and while it works without WXWIN_COMPATIBILITY_2_8 it paradoxically
doesn't work with it because this results in errors when wxADJUST_MINSIZE is
defined as 0 by wx itself.

Avoid this problem by only defining wxADJUST_MINSIZE ourselves if it hadn't
been defined yet.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64005 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775