Improve fftw m4 macro
First check if fftw functionality is already provided with the current flags/library. This is useful, e.g., when using MKL or when providing the FFTW flags via LDFLAGS/FCFLAGS/LIBS.
Also revert old commits that changed the fftw m4 macro, but broke the compilation when not specifying anything.
Improve fftw build system support
- I have checked that my code follows the Octopus coding standards