Add information about configure.sdkroot and macosx_deployment_target

[gallafent]

At present the guide has only a placeholder for macosx_deployment_target and related variables, and no mention at all of configure.sdkroot.

It would be very useful if these (and any others that are missing) had /at least/ placeholders (even without any meaningful documentation), so that they may be discovered. I recently /guessed/ at the existence of the configure.sdkroot variable, and this was exactly what I needed to use to get round a problem while testing a port which didn't compile! It would have been quicker if I could have seen it somewhere in the guide.

Also, an explanation of how to discover the list of valid variables directly from an installed port tree (for use when the documentation is outdated) would also be helpful.

[cooljeanius (Eric Gallager)]

Cc Me!

[mf2k (Frank Schima)]

markd has retired. See #44782.

[mascguy (Christopher Nielsen)]

Update as of this writing:

• configure.sdkroot is documented in the guide, but missing from the manpage for portfile.
• macosx_deployment_target is documented in the manpage, but missing from the guide.

[Christopher Nielsen <mascguy@…>]

In 9b28483ac923f06ebd76f84d37e7e9bcc1e36e60/macports-base (master):

manpages: portfile: document keyword configure.sdkroot

Fixes: #39083
See: #64329

[Christopher Nielsen <mascguy@…>]

In 07723aadf4cfb2907944cdc52cc855d919960542/macports-guide (master):

guide: keywords: document macosx_deployment_target

Fixes: #39083
See: #64329