Fix #13012 properly and don't set the SDK if CLT

Undoing parts of the hot fix 78b9e8548e771a59e382e6f13339664ec5498391.

The only thing missing was to check for `system "/usr/bin/xcrun -find make 1>/dev/null 2>&1"`
and then it's safe to call locate.

This commit restores the original functionality but without the risk for recursion
and improves the logic of `MacOS.locate`. See below.

To important changes in this commit:

- For Xcode _and_ CLT: don't add the SDK and leave things as before.
So if `MacOS.clt_installed?`, then no `SDKROOT` and `-L` and `-I`
directories are set in `ENV.macosxsdk`.

- Improved the logic for `MacOS.locate` for Xcode-only situations
by assuring that the xcode-select path is correct. This is done
by checking that `bin/make` exists and is executable. Otherwise it
was possible to set xcode-select to an empty dir.
This check is done in `MacOS.sdk_path` too.
We are now able to use Xcode wherever it is and can work even, if
xcode-select is set to invalid values. (Remember some users don't
have sudo access and that is needed to fix xcode-select).

Some minor whitespace fixes.
Minor backtick fix in doctor.rb's printout.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

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