fix spkg-configure.m4 for libffi

[dimpase]

libffi upstream hides headers behind ABI-specific directories. On gentoo this leads to failing ./configure checks for ffi.h, as
autoconf by itself is not aware of it.
See e.g. https://bugs.gentoo.org/643582
for details. A fix would be to call
pkg-config in case of failing checks for the header:

$ pkg-config --cflags libffi
-I/usr/lib64/libffi-3.3_rc0/include

More precisely, the right PKG_... autoconf macro.

On the other hand, this might be a gcc gentoo bug, as on Debian the location is still not /usr/include, but something including x86_64 in its name, yet the compiler finds the header just fine.

CC: @jdemeyer @embray

Component: packages: standard

Reviewer: Erik Bray

Issue created by migration from https://trac.sagemath.org/ticket/27175

[embray]

comment:1

This is already handled by #27114.

[embray]

Reviewer: Erik Bray