Don't call cbindgen recursively

To expand a crate, cbindgen calls cargo on that crate. cbindgen can be called from a build.rs
file. But if cargo is called again, it will also call cbindgen again and hence end up in an
endless recursion.

This commit makes sure that cbindgen isn't called again if it is already running.

You can verify this fix with this minimal example https://github.com/vmx/cbindgen-expand-bug

Fixes #347.

src/bindgen/cargo/cargo_expand.rs

@@ -83,6 +83,10 @@ pub fn expand(
         cmd.env("CARGO_TARGET_DIR", PathBuf::from(path).join("expanded"));
     }
 
+    // Set this variable so that we don't call it recursively if we expand a crate that is using
+    // cbindgen
+    cmd.env("_CBINDGEN_IS_RUNNING", "1");
+
     cmd.arg("rustc");
     cmd.arg("--lib");
     cmd.arg("--manifest-path");

src/bindgen/parser.rs

@@ -179,6 +179,13 @@ impl<'a> Parser<'a> {
     fn parse_expand_crate(&mut self, pkg: &PackageRef) -> Result<(), Error> {
         assert!(self.lib.is_some());
 
+        // If you want to expand the crate you run cbindgen on you might end up in an endless
+        // recursion if the cbindgen generation is triggered from build.rs. Hence don't run the
+        // expansion if the build was already triggered by cbindgen.
+        if std::env::var("_CBINDGEN_IS_RUNNING").is_ok() {
+            return Ok(());
+        }
+
         let mod_parsed = {
             if !self.cache_expanded_crate.contains_key(&pkg.name) {
                 let s = self