Get linker flags correctly from GHC ≥ 7.8

Get linker flags correctly from GHC ≥ 7.8

GHC 7.8 renamed "Gcc Linker flags" to "C compiler link flags" and "Ld
Linker flags" to "ld flags"
(https://ghc.haskell.org/trac/ghc/ticket/4862).

Without this corresponding fix in Cabal, the old-time package fails to
compile on Ubuntu yakkety amd64, where the system GCC now defaults to
-pie and GHC has

    $ ghc --info | grep -i pie
     ,("C compiler flags","-fno-PIE -fno-stack-protector")
     ,("C compiler link flags","-no-pie")
     ,("ld flags","-no-pie")

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
(cherry picked from commit c993a0c6660aa10d8c79ed0cc4961c59acfc91c8)

Gbp-Pq: Name get-linker-flags-correctly