How to build jpegsrc.v6b on 64-bit Linux machines
jpegsrc is no longer maintained and has not been updated since the Clinton administration. This is odd considering how critical this library is to so many applications.

If you are trying to build on a 64-bit Linux machine you will probably get this error during the ./configure step:

checking host system type... Invalid configuration `x86_64-unknown-linux-gnu': machine `x86_64-unknown' not recognized
The fix is simple -- just copy '/usr/share/libtool/config.guess' and '/usr/share/libtool/config.sub' to the same directory as the jpeg-6b ./configure script. You can also copy from from /usr/share/automake-1.9/.

cp /usr/share/libtool/config.guess .
cp /usr/share/libtool/config.sub .
./configure --enable-shared --enable-static

One site suggests that you also need tell the jpegsrc build where the 64-bit libdir is located:

make libdir=/usr/lib64
make libdir=/usr/lib64 install
That will change the directory where 'make install' puts the libjpeg library files, but you may not want that if you are installing libjpeg to a custom path (in other words, if you used the --prefix option for configure).