Utility: fix cross-compilation from Linux to Windows
Utility: fix cross-compilation from Linux to Windows

Only attempt to link against Backtrace if it is found.  Without this,
trying to cross-compile to Windows would try to link against
"Backtrace_LIBRARY-NOTFOUND.lib".

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@345569 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 53d62c8a5f480ec61cc372c7c0275502ef6123b6)

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb