STYLE: The FFTW libs should be co-located with ITK

When building FFTW internal to ITK, the fftw headers/libs
should be co-located with the ITK headers.  This patch
makes installation of FFTW and ITK consistent.

Change-Id: If2d4a429057c195bfb06056e8c68b8f827dbfe7a
Merge topic 'FixInstallationPathOfFFTW'

d07ee70 STYLE: The FFTW libs should be co-located with ITK