fix MacOS CRAN issue

[lschneiderbauer]

The CRAN MacOS build system seems to have fftw, but single precision support is not enabled, leading to a failed CRAN build for the MacOS platform.

• First attempt: submit anyway, since CRAN requirement state, that packages need to work for "at least two platforms". That attempt failed, the package initially accepted, but was archived after the grace period.

• Second attempt: try to modify the CRAN Mac-OS build system, see include single precision support in fftw R-macos/recipes#61

[lschneiderbauer]

positive macOS builder results:
https://mac.r-project.org/macbuilder/results/1736106958-949d71930686e96b/
https://mac.r-project.org/macbuilder/results/1736159623-58e1880408414635/