cd ~/tmp/
wget ftp://sourceware.org/pub/libffi/libffi-3.2.1.tar.gz
tar zxf libffi-3.2.1.tar.gz
cd ~/tmp/libffi-3.2.1
./configure --prefix=/home/user/tmp/libffi
make
make install
PKG_CONFIG_PATH=/home/user/tmp/libffi/lib/pkgconfig/ LD_LIBRARY_PATH=/home/user/tmp/libffi/lib/ pip install bcrypt

Finally add LD_LIBRARY_PATH information to Apache's start script (located in /home/user/webapps/foobar/apache2/bin)