macos: libffi needed by ctypes

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needed by ctypes. brew install:

...
libffi is keg-only, which means it was not symlinked into /usr/local,
because macOS already provides this software and installing another version in
parallel can cause all kinds of trouble.

For compilers to find libffi you may need to set:
  export LDFLAGS="-L/usr/local/opt/libffi/lib"
  export CPPFLAGS="-I/usr/local/opt/libffi/include"

For pkg-config to find libffi you may need to set:
  export PKG_CONFIG_PATH="/usr/local/opt/libffi/lib/pkgconfig"

In spite of this warning, after this opam install ctypes succeeds.