include $arch in your cmake find files

[ab1jx]

It should probably more like $ARCH, and I'm not sure what defines it.

But the finds for both fftw and itpp fail on a Raspberry Pi because the libraries installed in
/usr/lib/arm-linux-gnueabihf (Raspbian Jessie)

uname -A says
Linux pi2 4.9.35-v7+ #1014 SMP Fri Jun 30 14:47:43 BST 2017 armv7l GNU/Linux
so that won't help. Looking at stuff set in the environment I don't see exactly the right string anywhere. But there's probably a way. Looks like $HOSTTYPE$OSTYPE maybe, so
/usr/lib/$HOSTTYPE$OSTYPE

It finds the headers OK, just not the libraries. I changed your FindFFTW.cmake to have
FIND_LIBRARY(FFTW_LIBRARY
NAMES fftw3
PATHS ${FFTW_DIR}/libs
"${FFTW_DIR}\win32\lib"
/usr/lib/x86_64-linux-gnu
/usr/pkgs64/lib
/usr/lib64
/usr/lib
/usr/local/lib
/usr/lib/arm-linux-gnueabihf
NO_DEFAULT_PATH
)
And it worked.