use dlib for easier cross compiling

[Be-ing]

I'm trying to cross compile my application from x86-64 Linux to aarch64-unknown-linux-gnu to build it for my Pinephone and I am stumbling over Slint's dependency on fontconfig. I tried using an ARM64 container to cross compile, but using emulation with QEMU requires 2 hours to build! Slint currently uses the unsafe servo-fontconfig crate, so I opened an issue for that, but I think it would be better to improve this crate and switch Slint to using this.

The servo-fontconfig crate falls back to statically linking a vendored copy of libfontconfig if pkgconfig does not find it. This is problematic for two reasons. Most importantly, using a different copy of fontconfig than the system doesn't work as desired. Also, fontconfig requires a lot of C dependencies that need to be linked, which makes it difficult to get the vendored copy to build:

moire on  main [$!] is 📦 v0.1.0 via 🦀 v1.58.1
❯ pkg-config fontconfig --static --libs
-lfontconfig -lfreetype -lz -lbz2 -lpng16 -lm -lm -lz -lharfbuzz -lm -lglib-2.0 -lm -lpcre -lsysprof-capture-4 -pthread -lgraphite2 -lbrotlidec -lbrotlicommon -lxml2 -lz -llzma -lm

A solution to this would be using the dlib to let the user of the library either directly link fontconfig at build time or use dlopen to load it at runtime and remove the build time dependency. I recently helped implement this for the Rust JACK bindings and it makes cross compiling really easy. The Rust Wayland bindings use dlib as well and the X11 bindings have a similar option to use dlopen with the x11-dl crate.