Make package-dump work on other platforms

[finestructure]

This package is currently failing version discovery on the Swift Package Index, because swift package dump-package fails on Linux (where we run package checks):

/Users/sas/Projects/SPI/spi-server/SPI-checkouts/github.com-compnerd-swift-firebase/Package.swift:5:8: error: no such module 'WinSDK'
  3 | import PackageDescription
  4 | import Foundation
  5 | import WinSDK
    |        `- error: no such module 'WinSDK'
  6 |
  7 | let swiftc = "swiftc.exe".withCString(encodedAs: UTF16.self) { pwszCompiler in
error: invalid manifests at [<AbsolutePath:"/Users/sas/Projects/SPI/spi-server/SPI-checkouts/github.com-compnerd-swift-firebase">]

I'm not sure if the proposed way is the best way to fix it or if you're even interested in fixing it but I wanted to at least propose a solution.

Failing version discovery on the Swift Package Index means that we don't discover and show new releases on the package page.

[finestructure]

The package listing is here: https://swiftpackageindex.com/compnerd/swift-firebase

[compnerd]

Do we need an additional path for finding the include path for libdispatch? I'd rather have the package build on other platforms as well, it isn't Windows specific, this is just a limitation of SPM.

[finestructure]

I'm afraid I can't answer that - I just tried the minimal changes to make dump-package work on Linux/macOS 😅

[compnerd]

macOS doesn't require the flags, I'm not sure about Linux. I think we can simply conditionalize the flags for Windows.