Improve fftw m4 macro

Description

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.

Closes #244 (closed)

News snippet

Improve fftw build system support

Checklist

• I have checked that my code follows the Octopus coding standards