Migrate libc-test to 2018 edition

edition: cannot update to 2021 edition because of libc msrv.
cargo of rust 1.19 cannot understand edition key in cargo.toml.

(cherry picked from commit 4057a9a)

avoid rebuilding libc-test

no semver vendor = 'unknown'

(cherry picked from commit de2c8a7)

use rel path for semver

(cherry picked from commit d6d56f1)

(apply <#3718> to `main`)
[ resolve conflicts, squash for easier cherry-picks - Trevor ]

Author: tesuji

libc-test/Cargo.toml

@@ -1,6 +1,7 @@
 [package]
 name = "libc-test"
 version = "0.2.151"
+edition = "2018"
 authors = ["The Rust Project Developers"]
 license = "MIT OR Apache-2.0"
 build = "build.rs"

libc-test/build.rs

@@ -82,8 +82,7 @@ fn do_semver() {
     let target_env = env::var("CARGO_CFG_TARGET_ENV").unwrap();
 
     // `libc-test/semver` dir.
-    let mut semver_root = PathBuf::from(env::var("CARGO_MANIFEST_DIR").unwrap());
-    semver_root.push("semver");
+    let mut semver_root = PathBuf::from("semver");
 
     // NOTE: Windows has the same `family` as `os`, no point in including it
     // twice.
@@ -93,7 +92,10 @@ fn do_semver() {
     if family != os && os != "android" {
         process_semver_file(&mut output, &mut semver_root, &family);
     }
-    process_semver_file(&mut output, &mut semver_root, &vendor);
+    // We don't do semver for unknown targets.
+    if vendor != "unknown" {
+        process_semver_file(&mut output, &mut semver_root, &vendor);
+    }
     process_semver_file(&mut output, &mut semver_root, &os);
     let os_arch = format!("{}-{}", os, arch);
     process_semver_file(&mut output, &mut semver_root, &os_arch);
@@ -141,6 +143,9 @@ fn process_semver_file<W: Write, P: AsRef<Path>>(output: &mut W, path: &mut Path
 }
 
 fn main() {
+    // Avoid unnecessary re-building.
+    println!("cargo:rerun-if-changed=build.rs");
+
     do_cc();
     do_ctest();
     do_semver();