fftw: fix fortran

Closes #20727.
Show conflicting files on install if linking fails

Closes #18303.
Closes #20726.
Show tap/path for non-core formulae on build failure

Closes #16373.
Closes #20725.
Use 'path or URL' as tap for non-tap, non-core formulae
Misplaced manpages fail brewbot

Closes #20711.
Add :fortran requirement
Use :fortran

Closes #20731.
Closes #13621.
Update audit for Fortran dependency
Add Fortran ENV helpers
Use Fortran ENV helpers
Add 'From:' to GitHub url for formula

Closes #20724.