libffi does not honor includedir

Hi,

I am currently in the process of building glib and discovered that libffi as a subproject does not honor the includedir and always places the header files in an include directory directly below prefix. However, the includedir path information in the libffi pkgconfig file is correctly pointing to the specified includedir.

This behavior should be corrected with this patch: patchfile.patch

However, I am new to using meson and have no idea whether this patch breaks something in other projects or if there is a superordinate prefix specifically for subprojects that should be used.