buildtools: the buildtools dir is in fact a prefix

Consider the buildtools install directory as a prefix directory,
that is, install buildtools in prefix/bin/, not in prefix/.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 This is NOT the official version of dialog.  This version has been

 significantly modified from the original.  It is for use by the Linux

 kernel configuration script.  Please do not bother Savio Lam with 

 questions about this program.