Fixes build failure with gcc-5.3 on i586.
Enabled gsl and fftw by default, since they are both in Slackware.
Moved from /usr/ROOT to /opt/ROOT.
Script cleanups.
(changed PREFIX from /usr to /usr/ROOT -with profile.d scripts
and ld.so.conf- moved python stuff, use system libs --ponce)
Signed-off-by: Matteo Bernardini <ponce@slackbuilds.org>