Add a WX_APPNAME_DATA_DIR hack for wxStandardPaths::GetDataDir().

Applications using wxStandardPaths::GetDataDir() to find their files under
Unix can't be ran without being installed as they look for their data files
under $prefix/share/appname. Make it possible to override this location by
setting WX_APPNAME_DATA_DIR environment variable to allow running them without
installation.

Notice that this shouldn't present any security risk unless the application is
SUID (which would be a very bad idea anyhow).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62337 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/stdpaths.h

@@ -103,11 +103,20 @@ public:
     /**
         Return the location of the applications global, i.e. not user-specific,
         data files.
+
         Example return values:
         - Unix: @c prefix/share/appinfo
         - Windows: the directory where the executable file is located
         - Mac: @c appinfo.app/Contents/SharedSupport bundle subdirectory
 
+        Under Unix (only) it is possible to override the default value returned
+        from this function by setting the value of @c WX_APPNAME_DATA_DIR
+        environment variable to the directory to use (where @c APPNAME is the
+        upper-cased value of wxApp::GetAppName()). This is useful in order to
+        be able to run applications using this function without installing them
+        as you can simply set this environment variable to the source directory
+        location to allow the application to find its files there.
+
         @see GetLocalDataDir()
     */
     virtual wxString GetDataDir() const;