Disable modern build system for Xcode 10

[mlegenhausen]

Just as requested in #378 I provide my minimal solution.

Platforms affected

iOS 12 with Xcode 10 GM

What does this PR do?

It disables the modern build system of Xcode 10 via a undocumented command line flag for xcodebuild.

What testing has been done on this change?

cordova build ios and cordova run ios (emulator)

Checklist

• Reported an issue in the JIRA database
• Commit message follows the format: "CB-3232: (android) Fix bug with resolving file paths", where CB-xxxx is the JIRA ID & "android" is the platform affected.
• Added automated test coverage as appropriate for this change

[dpogue]

Our hope is to make the next major version of cordova-ios compatible with the new build system, rather than disabling it.

If you're running into issues with Xcode 10 and need to disable it for your existing projects, there are some workarounds documented here.

[geocolumbus]

We are building using "buildFlag": ["-UseModernBuildSystem=0"] option in the build.json file with XCode 10 for our iOS12 phones. So this is an excellent workaround. But maybe that flag (and the old build system) will vanish in XCode 11. Anyway, this is a good work around until cordova-ios gets sorted out. We are also mourning the loss of UIWebView and all the problems of migrating to WKWebView for our enterprisy authentication flow. Apple is a handful.

[dpogue]

We've merged support for the new build system in preparation for the next major version of cordova-ios, so I'm going to close this. Thanks for your contribution!