Use Darwin libtool's -no_warning_for_no_symbols if available to silence the "has no symbols" link warning
Use Darwin libtool's -no_warning_for_no_symbols if available to silence the "has no symbols" link warning

Building compiler-rt on Darwin produces dozens of meaningless warnings about object files having no symbols during static archive creation. This is very intentional as compiler-rt uses #ifdefs to conditionally compile platform-specific code, and we even have a .cpp source file that only contains static asserts to make sure the environment is configured right. On Linux, this situation is fine and no warning is produced. This patch adds a libtool version detection and if it's new enough, we'll use the -no_warning_for_no_symbols flag that suppresses this warning. Build logs should be much cleaner now!

Differential Revision: https://reviews.llvm.org/D27119



git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@288640 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm