ref_cpu fixes
ref_cpu fixes

* ref_cpu: use sensible default for FFTW library env vars, and make them optional

* ref_cpu: fix backwards pointer check on dlclose
ROCmSoftwarePlatform/rocFFT,ROCmSoftwarePlatform/rocFFT,ROCmSoftwarePlatform/rocFFT,ROCmSoftwarePlatform/rocFFT