Minor changes in the configure's options

[Sylvestre Ledru]

Hello,

After the build-help and build-localization, I changed the default
behaviour in the configure.
Now, by default, all options are activated and can be disabled by hand
if you don't want to build it.

The options are:
* --without-pvm
* --without-fftw
* --without-umfpack

The purpose of this modification is to have consistent packages of
Scilab in term of spectrum.

Sylvestre