move abi checks to a feature and build the C code at build time

The abi tests are now conditional to the abi-tests feature,
when that is enabled, the C code for the abi tests is now built
at compile time, using the cc crate rather than having our own
Compiler abstraction and having to compile stuff at runtime and
deal with temp directories etc.

src/codegen/sys/build.rs

@@ -1,5 +1,5 @@
 use super::collect_versions;
-use crate::{codegen::general, env::Env, file_saver::save_to_file};
+use crate::{codegen::general, env::Env, file_saver::save_to_file, library::MAIN_NAMESPACE};
 use log::info;
 use std::io::{Result, Write};
 
@@ -30,10 +30,14 @@ pub fn generate(env: &Env) {
 
 #[allow(clippy::write_literal)]
 fn generate_build_script(w: &mut dyn Write, env: &Env, split_build_rs: bool) -> Result<()> {
+    let ns = env.library.namespace(MAIN_NAMESPACE);
+    let package_name = ns.package_name.as_ref().expect("Missing package name");
+
     if !split_build_rs {
         general::start_comments(w, &env.config)?;
         writeln!(w)?;
     }
+
     writeln!(
         w,
         "{}",
@@ -55,10 +59,43 @@ fn main() {} // prevent linking libraries to avoid documentation failure
 
 #[cfg(not(feature = "dox"))]
 fn main() {
-    if let Err(s) = system_deps::Config::new().probe() {
+    let libs = system_deps::Config::new().probe();
+    if let Err(s) = libs {
         println!("cargo:warning={}", s);
         process::exit(1);
     }
+
+    #[cfg(feature = "abi-tests")]
+    {
+        let libs = libs.unwrap();
+"##
+    )?;
+
+    write!(
+        w,
+        "        let includes = libs.get(\"{}\").unwrap().include_paths.clone();",
+        package_name
+    )?;
+
+    write!(
+        w,
+        "{}",
+        r##"
+
+        let mut cc = cc::Build::new();
+
+        cc.flag_if_supported("-Wno-deprecated-declarations");
+        cc.flag_if_supported("-std=c11"); // for _Generic
+
+        cc.file("tests/constant.c");
+        cc.file("tests/layout.c");
+
+        for i in includes {
+            cc.include(i);
+        }
+
+        cc.compile("cabitests");
+    }
 }
 "##
     )

src/codegen/sys/cargo_toml.rs

@@ -83,14 +83,13 @@ fn fill_in(root: &mut Table, env: &Env) {
 
     {
         let build_deps = upsert_table(root, "build-dependencies");
+        set_string(build_deps, "cc", "1.0.0");
         set_string(build_deps, "system-deps", "2.0");
     }
 
     {
         let dev_deps = upsert_table(root, "dev-dependencies");
         set_string(dev_deps, "shell-words", "1.0.0");
-        set_string(dev_deps, "tempfile", "3");
-        unset(dev_deps, "tempdir");
     }
 
     {
@@ -115,6 +114,7 @@ fn fill_in(root: &mut Table, env: &Env) {
                     .collect(),
             ),
         );
+        features.insert("abi-tests".to_string(), Value::Array(Vec::new()));
     }
 
     {