Fix issues with bundled pjproject cached download.

Previously when testing I had a preexisting makeopts in ASTTOPDIR.  The
ordering of configure.ac causes --with-externals-cache to be processed
after third-party configure.  In cases where the Asterisk clone is
cleaned it would cause pjproject to be downloaded to /tmp.  This
moves processing of the externals cache and sounds cache to happen
before third-party configure.

This also addresses a possible issue with the third-party Makefile.  If
TMPDIR is set by the environment it would override the path given to
--with-externals-cache.

ASTERISK-26416

Change-Id: Ifab7f35bfcd5a31a31a3a4353cc26a68c8c6592d