ghc-7.8.4: re-implement "fix build on Darwin" to avoid re-builds on Linux

Originally, I thought that I can commit a "clean" patch -- even if it
triggers re-builds -- because those re-builds were triggered by the
ncurses patch to GHC anyway . That patch had to be reverted, though, so
now I'm rewriting this patch to avoid re-builds on Linux.

What a mess. :-(

pkgs/development/compilers/ghc/7.4.2-binary.nix

@@ -62,9 +62,8 @@ stdenv.mkDerivation rec {
      '' else "");
 
   configurePhase = ''
-    ./configure --prefix=$out \
-      --with-gmp-libraries=${gmp}/lib --with-gmp-includes=${gmp}/include \
-      ${stdenv.lib.optionalString stdenv.isDarwin "--with-gcc=${./gcc-clang-wrapper.sh}"}
+    ./configure --prefix=$out --with-gmp-libraries=${gmp}/lib --with-gmp-includes=${gmp}/include \
+      ${if stdenv.isDarwin then "--with-gcc=${./gcc-clang-wrapper.sh}" else "--with-clang"}
   '';
 
   # Stripping combined with patchelf breaks the executables (they die