Error: Drush !version is not supported

Trying to install Aegir 7.x-3.x with Drush 5.x fails with the following error message:

drush version 5.10.0
Cannot determine release date for drush [18.47 sec, 9.63 MB]                                                                                  [notice]
Drush !version is not supported [18.47 sec, 7.18 MB]                                                                                       [error]

I traced this issue to line 86 of provision/platform/drupal/install_7.inc:

switch (substr(drush_core_version(), 0, 1)) {

That doesn't work because of the differences in drush_core_version() in Drush 5.x and Drush 6.x+.

In 5.x, it prints the Drush version using drush_print():

http://api.drush.org/api/drush/commands%21core%21core.drush.inc/function...

In 6.x+, it returns the DRUSH_VERSION constant:

http://api.drush.org/api/drush/commands%21core%21core.drush.inc/function...

Because 5.x's drush_core_version() doesn't actually return a value (it prints it), the switch statement at line 86 on install_7.inc cannot work with it.

I was able to bypass this issue and get Aegir installed by changing line 86 to:

switch (substr(DRUSH_VERSION, 0, 1)) {

However, I don't think that's a good permanent fix. drush_core_version() is the ideal approach... it just doesn't work in 5.x. So we need to do it another way.