This reverts commit 000d95670b and commit
d764796e0c to fix arm cross build and
x86-64 linux.
Let's use the core::ffi types when fontconfig-sys switches to it.
This allows setting the RUST_FONTCONFIG_DLOPEN environment variable
to dlopen fontconfig at runtime rather than linking it at build
time. This is helpful for cross compiling to Linux, particularly
because fontconfig has lots of C dependencies. Building a vendored
copy of fontconfig does not work as expected:
https://github.com/slint-ui/slint/issues/88
