PIP install unable to find ffi.h even though it recognizes libffi

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 recognizes that it is installed; however, when trying to install pyOpenSSL, pip states that it cannot find file 'ffi.h'. I know both thatffi.h exists as well as its directory, so how do I go about closing this gap between ffi.h and pip?

You can use CFLAGS (and LDFLAGS or various other compiler and linker options) in front of the pip command (ditto for setup.py):

Something similar to the following should work:

    CFLAGS=-I/usr/include/libffi/include pip install pyOpenSSL

From: stackoverflow.com/q/31508612