Command explanations
./configure --prefix= : We do this because
shells should be installed in the /bin directory, not /usr/bin.
cp tcsh.man /usr/share/man/man1/tcsh.1
:tcsh doesn't install its man-page correctly, so we do it
manually.
ln -s tcsh /bin/csh : The FHS states that
if there is a C shell installed, there should be a symlink from
/bin/csh to it. This creates that symlink.