Update build instructions for new autoconf, GitHub

The Linux build machine has been upgraded to autoconf 2.69, automake
1.15, m4 1.4.17, and libtool 2.4.6, so it is no longer necessary to
recommend running autoreconf prior to building the source, if one is
building from an official source tarball (as opposed to from a git
sandbox.)  Also, there is no SVN repository anymore (oops.)
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