Fixes uninitialized var, static libs, CLI options.

[tdd11235813]

• cmake: GEARSHIFFT_BACKEND_FFTW_THREADS -> GEARSHIFFT_INTERNAL_FFTWX_THREADED
• FFTWWrappers/MKL does not implement parameters for rigor & timelimit
  • so do not show CLI options for that
• Fixes and moves non-MKL libs in MKL cmake variable.
• show messages only when not set to quiet (FindFFTWWrappers.cmake)
• Only restrict suffix for static libs (also in FindFFTW.cmake)

@emmenlau Can you test this PR on your systems? I changed some logic from your recent PRs.
edit: refers to #140

[psteinb]

works for me with xcode AppleClang 9.0.0.9000034, boost: stable 1.69.0 (bottled) and fftw: stable 3.3.8 (bottled). The latter 2 from homebrew. The more we support other OSes, the more we need to think about adding other OSes to our .travis build. Think about a multi-OS build matrix.

[psteinb]

can this be merged?

[tdd11235813]

let me test this again. If it will work, then we can merge it.
Edit: going to rebase ...

[tdd11235813]

tested on taurus: if intel mkl and intel compiler versions do not match it seems this will then lead to a stupid segfault after program has finished and cleaned up. This is also present on previous versions.

Working module setup on taurus:

# in scs5 env
module load imkl/2019.1.144-iimpi-2019a Boost/1.70.0-iimpi-2019a icc/2019.1.144-GCC-8.2.0-2.31.1
CXX=icc cmake ..

with this modules it worked, but I could not test all cases (GNU MKL wrapper?). Can you also test again?

[psteinb]

I'll test that next week.

[tdd11235813]

k, np, I do not touch this PR.