New syntax: pkg-flags

I've added new groveler syntax, pkg-flags, that uses pkg-config --cflags to find the cc-flags to add. As an example, I added this syntax to libffi-unix.lisp.

Background: On Gentoo, libffi installs header files in /usr/lib64/libffi-<version>/include. This makes compilation of cffi-libffi fail unless editing libffi-unix.lisp, and will fail when libffi is upgraded. This fixes the problem for me, but also, pkg-flags may be a useful addition to the groveler syntax.