PR/17383: fix gcc in-srcdir build (take 2)

[Paolo Bonzini]

The attached patch creates the host-subdir only if gcc is found in the
tree.  It is even a bit simpler than the original because the topdir is
found more easily, replacing with .. every element of the path from the
topdir to the builddirs.

It has passed in-srcdir and out-of-srcdir builds of gcc (all languages,
powerpc-apple-darwin7.8.0) and binutils+gdb (i686-pc-linux-gnu).  Ok for
gcc/src?

Paolo

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