Description of problem:
The headers are stored in %{_libdir}/%{name}-%{version}/include/

This is wrong. They should be included in %{_includedir}.

Version-Release number of selected component (if applicable):
[root@ankur ~]# repoquery libffi-devel 
libffi-devel-0:3.0.9-2.fc15.i686
libffi-devel-0:3.0.9-2.fc15.x86_64


[root@ankur ~]# repoquery -l libffi-devel
/usr/lib/libffi-3.0.9
/usr/lib/libffi-3.0.9/include
/usr/lib/libffi-3.0.9/include/ffi.h
/usr/lib/libffi-3.0.9/include/ffitarget.h
/usr/lib/libffi.so
/usr/lib/pkgconfig/libffi.pc
/usr/share/info/libffi.info.gz
/usr/share/man/man3/ffi.3.gz
/usr/share/man/man3/ffi_call.3.gz
/usr/share/man/man3/ffi_prep_cif.3.gz
/usr/lib64/libffi-3.0.9
/usr/lib64/libffi-3.0.9/include
/usr/lib64/libffi-3.0.9/include/ffi.h
/usr/lib64/libffi-3.0.9/include/ffitarget.h
/usr/lib64/libffi.so
/usr/lib64/pkgconfig/libffi.pc
/usr/share/info/libffi.info.gz
/usr/share/man/man3/ffi.3.gz
/usr/share/man/man3/ffi_call.3.gz
/usr/share/man/man3/ffi_prep_cif.3.gz