1. Get the path to the pwd binary (/bin/pwd usually) from configure.
2. Use that path to run pwd in all path variables set in Makefile.config.in
The hope is that these changes will resolve symlinks to physical paths. This
should work on all platforms where the binary pwd defaults to printing
physical paths. The shell version of pwd generally doesn't (it will print
the symlink path).

llvm-svn: 29381