fftw: update to 3.3.7.

FFTW 3.3.7:

* Experimental support for CMake.

  The primary build mechanism for FFTW remains GNU autoconf/automake.
  CMake support is meant to offer an easy way to compile FFTW on
  Windows, and as such it does not cover all the features of the
  automake build system, such as exotic cycle counters,
  cross-compiling, or build of binaries for a mixture of ISA's
  (e.g., amd64 vs amd64+avx vs amd64+avx2).  Patches are welcome.

* Fixes for armv7a cycle counter.
* Official support for aarch64, now that we have hardware to test it.
* Tweak usage of FMA instructions in a way that favors newer processors
  (Skylake and Ryzen) over older processors (Haswell).
* tests/bench: use 64-bit precision to compute mflops.

FFTW 3.3.6-pl2:

* Bugfix: MPI Fortran-03 headers were missing in FFTW 3.3.6-pl1.

(wiz)