The download process now has a timeout which will cause wget to retry
if it stops retrieving data for 5 seconds and fetch and curl to timeout
if the whole retrieval take smore than 30 seconds.

If the tarball retrieval works, the MD5SUM file is retrieved from
the downloads site and the md5 checksum is verified.

If either the tarball retrieval or MD5SUM retrieval fails, or the
checksums don't match, the entire process is retried once.  If it
fails again, any incomplete tarball is deleted.

.DELETE_ON_ERROR: was also added to the Makefile.  Not only does
this delete the tarball on failure, it till also delete corrupted
library files from the pjproject source directory should they
fail to build correctly.

Tested all the way back to FreeBSD 9, CentOS 6, Debian 6 and
Ubuntu 14.

Change-Id: Iea7d33b96a31622ab1b6e54baebaf271959514e1