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.

include/wx/utils.h

@@ -162,7 +162,7 @@ WXDLLIMPEXP_BASE wxLinuxDistributionInfo wxGetLinuxDistributionInfo();
 WXDLLIMPEXP_BASE wxString wxNow();
 
 // Return path where wxWidgets is installed (mostly useful in Unices)
-WXDLLIMPEXP_BASE const wxChar *wxGetInstallPrefix();
+WXDLLIMPEXP_BASE wxString wxGetInstallPrefix();
 // Return path to wxWin data (/usr/share/wx/%{version}) (Unices)
 WXDLLIMPEXP_BASE wxString wxGetDataDir();