cmake: try find system-wide FFTW3 before self-compiling it

The policy is to first try to find a system installation of FFTW3 (i.e.
searching for a fftw3.h and libfftw3.so). Iff this fails, CMake will
download and compile a static library of FFTW3 on its own in the build
directory of PFASST++.

this should fix #54
travis: make travis use Ubuntu's libfftw3
Merge pull request #55 from torbjoernk/feature/cmake-fftw

Let CMake find and use preinstalled FFTW3