build: use build options to configure terminal C ABI mode

Fixes various issues:

- C ABI detection was faulty, which caused some Zig programs to use
the C ABI mode and some C programs not to. Let's be explicit.

- Unit tests now tests C ABI mode.

- Build binary no longer rebuilds on any terminal change (a regression).

- Zig programs can choose to depend on the C ABI version of the terminal
  lib by using the `ghostty-vt-c` module.

1 files changed, 9 insertions, 0 deletions

diff --git a/build.zig b/build.zig
index 008fc849e..62fa77511 100644
--- a/build.zig
+++ b/build.zig
@@ -255,6 +255,15 @@ pub fn build(b: *std.Build) !void {
         });
         const mod_vt_test_run = b.addRunArtifact(mod_vt_test);
         test_lib_vt_step.dependOn(&mod_vt_test_run.step);
+
+        const mod_vt_c_test = b.addTest(.{
+            .root_module = mod.vt_c,
+            .target = config.target,
+            .optimize = config.optimize,
+            .filters = test_filters,
+        });
+        const mod_vt_c_test_run = b.addRunArtifact(mod_vt_c_test);
+        test_lib_vt_step.dependOn(&mod_vt_c_test_run.step);
     }
 
     // Tests