ghostty: link against system fontconfig to fix crashes

[maxib0n]

Summary

Ghostty statically bundles its own copy of fontconfig. Because the resulting binary is a PIE executable built without hiding those symbols, it globally exports ~27 Fc* functions. GTK4/Pango load the system libfontconfig.so.1 into the same process, and due to ELF symbol interposition, calls made from inside libfontconfig.so.1 (and other GTK-side consumers, e.g. glycin's SVG loader, see ghostty-org/ghostty#12555) get redirected to Ghostty's bundled copy instead, corrupting state shared between the two fontconfig instances.

This causes reproducible SIGSEGV/SIGABRT crashes (invalid free in FcPatternDestroy/FcFontSetDestroy, or segfaults in FcFontSort). The easiest reliable trigger is moving a Ghostty window between two monitors with different scale factors, which forces a font grid rebuild through fontconfig (Surface.contentScaleCallback → setFontSize → SharedGridSet.ref → Fontconfig.discover).

Passing -fsys=fontconfig to zig build makes Ghostty dynamically link the system fontconfig instead of statically bundling its own, so there's a single fontconfig instance in the process.

Verification

On the currently published ghostty-1.3.1-2.fc44 build:

$ nm -D /usr/bin/ghostty | grep -c '^[0-9a-f]* T Fc'
27
$ readelf -d /usr/bin/ghostty | grep -i needed | grep -i font
(nothing — fontconfig is not a declared dependency of the binary itself)

8 coredumps collected over the past week, 6 with matching stack traces through contentScaleCallback → setFontSize → SharedGridSet.ref → FcFontSetDestroy/FcPatternDestroy or FcFontSort.

Rebuilt locally with -fsys=fontconfig added (same Zig 0.15.2, same build flags otherwise):

$ nm -D ghostty | grep -c '^[0-9a-f]* T Fc'
0
$ readelf -d ghostty | grep -i needed | grep -i font
 0x0000000000000001 (NEEDED)  Shared library: [libfontconfig.so.1]

Ran the patched binary and repeatedly moved the window between the two monitors that reliably crashed the unpatched build (scale 1.25 vs scale 1) — no crash.

Test plan

• zig build succeeds with -fsys=fontconfig added (209/209 steps)
• Confirmed 0 exported Fc* symbols vs 27 before
• Confirmed libfontconfig.so.1 is a proper NEEDED entry
• Manually reproduced the crash on the unpatched binary, then confirmed the patched binary survives the same repro steps