I have installed libffi on my Linux server as well as correctly set the PKG_CONFIG_PATH environment variable to the correct directory, as pip
libffi
PKG_CONFIG_PATH
pip
pip packages usually don't use pkg-config. Therefore, you should set CFLAGS and LDFLAGS manually:
pkg-config
CFLAGS
LDFLAGS
CFLAGS=$(pkg-config --cflags libffi) LDFLAGS=$(pkg-config --libs libffi) pip install pyOpenSSL