Install public headers in the standard path?

[Chi-Hsuan Yen <yan12125@gmail.com>]

Hello libffi enthusiasts,

Currently libffi public headers are installed in
$PREFIX/lib/libffi-3.x.y/include. The only reliable approach to get
the include path is via pkg-config. There are some disadvantages:

1. pkg-config is tricky in terms of cross-compilation. To cross-build
libffi clients, PKG_CONFIG_LIBDIR, PKG_CONFIG_SYSROOT_DIR,
PKG_CONFIG_ALLOW_SYSTEM_{CFLAGS,LIBS} may be involved to make it
working

2. pkg-config does not come with macOS. Recently CPython is seeking
for chances to drop bundled libffi copy. [1] However, CPython core
developers are against including additional tools other than
XCode/Command Line Tools in CPython build time requirements

There are some more discussions here:
https://github.com/libffi/libffi/issues/258

As a result, I suggest to move header files to conventional
$PREFIX/include, so that libffi clients can have simpler build
systems.

Here's my implementation: https://github.com/libffi/libffi/pull/288

Any ideas?

Best,

Yen, Chi-Hsuan

[1] http://bugs.python.org/issue28491