Minor error in autogen help text for nasm

[Christian Lohmaier]

Hi Chris,

On Fri, Mar 31, 2017 at 11:21 PM, Chris Gill <chrisgill948 at gmail.com> wrote:
>
> When I first ran autogen.sh I did not have yasm or nasm installed.  I got a
> message saying "consult
> http://svn.code.sf.net/p/libjpeg-turbo/code/trunk/BUILDING.txt".  But that
> is a broken link.
>
> I think a better URL to cite might be
> https://github.com/libjpeg-turbo/libjpeg-turbo/blob/master/BUILDING.md

Thx, fixed that on master

ciao
Christian