* Don't bother specifying a download directory to CMake ExternalProject
* Teach CMake to build pure Xcode projects as dependencies
This allows downstream code (like log_apple.mm) to consume this for
testing within the CMake build without requiring a CMake-native build
for these components.
This makes integrating these components into the cmake build essentially
free from the point of view of the consumed component.
* Get the CMake build semi-working on Linux again
Many prebuilt versions of cmake on Linux lack the ability to download
over https so use git to get googletest.
Don't attempt to build FirebaseCore on Linux; there's no xcodebuild.
Note the build is still ultimately broken because we don't yet have an
alternative to arc4random on Linux but at least this is no more broken
than it was before.
