[PATCH] Fix in-srcdir build of gcc

[DJ Delorie]

> While this can be even expected for gcc, which anyway is almost always 
> built out-of-srcdir, it may violate the "principle of least surprise" 
> for binutils or gdb.

As I am a proponent of "principle of least surprise" (and have been
keeping build-in-srcdir mostly working, with little reward) I'm going
to refrain from commenting.  I'll leave it up to the other build
maintainers to decide on this one.