We now have various flags that need to be passed to gcc, but only if
the local gcc version supports them.  So instead of wiring this
knowledge into ghc when it is built, we now put these "extra gcc
flags" into a text file in $libdir, extra-gcc-flags, which is created
by configure.  It is also created by the configure script of a binary
distribution, so a binary dist is independent of the gcc version used
to build it.

Hopefully I got the binary-dist stuff right, but binary dists aren't
currently working so we'll need to come back and check this.