test: extract the `genmerges` out of test-merge-combination.t This open the way to splitting this slow test in multiple ones. Differential Revision: https://phab.mercurial-scm.org/D9767

fn main() {
    cc::Build::new()
        .warnings(true)
        .file("src/sendfds.c")
        .file("src/sighandlers.c")
        .compile("procutil");
}