Clarify license terms

shape4D executable depends on FFTW. FFTW is available under two licenses:
* (1) GPLv2 or any later version.
* (2) Commercial license removing the GPL restrictions.

For more details, see  http://www.fftw.org/doc/License-and-Copyright.html

See  also https://github.com/jamesfishbaugh/shape4D/blob/master/SuperBuild/External_FFTW.cmake

cc @bpaniagua