enable subdir-objects for automake
enable subdir-objects for automake

Enable subdir-objects to silence warning caused by the addition
of linux_stubs.c to src/shims.  This seems like the most contained
fix to allow a .c file in the shims subdir without warnings.

frankeh/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,frankeh/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,frankeh/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,frankeh/swift-corelibs-libdispatch