configure: support directories containing .o

Fixes http://code.google.com/p/webm/issues/detail?id=96

The regex which postprocesses the gcc make-deps (-M) output was too
greedy and matching in the dependencies part of the rule rather than
the target only. The patch provided with the issue was not correct, as
it tried to match the .o at the end of the line, which isn't correct
at least for my GCC version. This patch matches word characters
instead of .*

Thanks to raimue and the MacPorts community for isolating this issue.

Change-Id: I28510da2252e03db910c017101d9db12e5945a27