build-sys: accommodate gcc-4.9.0 link-time optimization (LTO) changes

systemd fails to build (symbols not found/resolved during cgls link step)
under gcc-4.9.0 due to link-time optimization (lto) changes, in particular
from gcc-4.9.0/NEWS:

  + When using a linker plugin, compiling with the -flto option
    now generates slim objects files (.o) which only contain
    intermediate language representation for LTO. Use
    -ffat-lto-objects to create files which contain additionally
    the object code. To generate static libraries suitable for LTO
    processing, use gcc-ar and gcc-ranlib; to list symbols from a
    slim object file use gcc-nm. (Requires that ar, ranlib and nm
    have been compiled with plugin support.)

Both -flto and -ffat-lto-objects are now needed when building and linking
against static libs w/LTO.