Solve fatal error: ‘FFI. H’ file not found

Error installing Rumps in Python:

Modules/objc/libffi_support.h:4:10: fatal error: 'ffi.h' file not found
#include "ffi.h"
1 error generated.
error: Setup script exited with error: command 'cc' failed with exit status 1

The reason is the lack of libffi library, using BREW can be installed.

$ brew install pkg-config libffi
$ export PKG_CONFIG_PATH=/usr/local/Cellar/libffi/3.0.13/lib/pkgconfig/
$ pip install bcrypt

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