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

[DJ Delorie]

> So, I'd approve this change if it wasn't for DJ's objection.

I object to ./configure not building in $srcdir on principle.  I
didn't say I objected to this patch specifically, but I didn't feel it
would be fair for me to review it with my bias.  Also, since there
seems to be little reward for maintaining build-in-srcdir, I feel my
arguments for keeping that are weak.

So at this point, I'm rather ambivalent about whether it's changed or
not, so I abstained.