I need to download the macOS 10.12 SDK to build and run ispwDownloader (because there were no built downloads).
How can I download and install the SDK? Searching the Web has not been helpful with the solution.
You'll have to download and run the compatible version of Xcode which included macOS 10.12 SDK, on compatible version of macOS.
macOS 10.12 SDK was first introduced in Xcode 8.0. The last macOS 10.12 SDK was macOS 10.12.4 SDK which was released with Xcode 8.3. Xcode 8.3 saw three bug fix releases with the last one being Xcode 8.3.3.
You'll need to download both the desired version of Xcode, and the compatible release of macOS which can run the said Xcode release. The information can be obtained from the linked table.
You can download Xcode 8.3.3 from Apple Developer downloads. Here's the direct link for the download. You may need to login with your Apple ID to initiate the download.
To run Xcode 8.3.3 you'll need either macOS Sierra 10.12 or macOS High Sierra 10.13. The latest release for them can be downloaded from the Mac App Store.
@Nimesh Neema, the SDKs are stored in
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/, and I got the 10.12 SDK from here.