#42 Allow fftw3 support to be explicitly disabled

[Marek Szuba]

At the moment, ngspice builds with fftw3 support if configure finds the library and its header files. This may result in a hidden dependency on that library for FreeBSD, Gentoo Linux etc. users, and makes life slightly harder for binary-package builders.

The attached patch adds a new configure option, --with-fftw3, which can be used to explictly prevent that script from looking for fftw3. It defaults to on so for anyone who does not use it, ngspice configure ought to behave as before.

[Dietmar Warning]

Applied the patch because it is useful too for debugging purpose.