Don't crash if WXPREFIX env. variable is set

Change wxGetInstallPrefix() to return a string instead of const
wxChar*. The latter was incorrectly obtained from a temporary string if
WXPREFIX was set. While it's possible to fix in a backward compatible
manner without changing the function's signature, it's not worth the
effort for something pretty obscure and used mostly internally.

docs/changes.txt

@@ -105,6 +105,8 @@ Changes in behaviour not resulting in compilation errors
 - wxFileDialog::GetPath() and wxFileDialog::GetFilename() now assert and return
   an empty string if called on dialogs with the wxFD_MULTIPLE style.
 
+- wxGetInstallPrefix() now returns wxString.
+
 
 Changes in behaviour which may result in build errors
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