configure: Check cache for valid pjproject tarball before downloading.

On a fresh Asterisk source directory, the bundled pjproject tarball is
unconditionally downloaded even if the tarball is already in a specified
cache directory.

* Made check if the pjproject tarball is valid in the cache directory
before downloading the tarball on a fresh source directory.

Change-Id: Ic7ec842d3c97ecd8dafbad6f056b7fdbce41cae5