Fix ./validate with GHC 7.6 as bootstrap compiler.

7.6 deprecated the Control.Concurrent.QSem module (to be removed later,)
but according to Patrick it was actually un-deprecated. As a result,
validate fails if your bootstrap compiler is 7.6, since it throws a
DEPRECATED warning.
Signed-off-by: Austin Seipp <austin@well-typed.com>

compiler/main/GhcMake.hs

 {-# LANGUAGE ScopedTypeVariables #-}
+{-# OPTIONS_GHC -fno-warn-warnings-deprecations #-}
+-- NB: we specifically ignore deprecations. GHC 7.6 marks the .QSem module as
+-- deprecated, although it became un-deprecated later. As a result, using 7.6
+-- as your bootstrap compiler throws annoying warnings.
 
 -- -----------------------------------------------------------------------------
 --