Don't use posix_spawn() if PATH was modified in the environment.

The expected behavior is that the environment's PATH should be used
to find the process.  posix_spawn() could be used if we iterated
PATH to search for the binary to execute.  For now just skip
posix_spawn() if PATH is modified.

src/libstd/sys/unix/process/process_common.rs

@@ -184,6 +184,9 @@ impl Command {
         let maybe_env = self.env.capture_if_changed();
         maybe_env.map(|env| construct_envp(env, &mut self.saw_nul))
     }
+    pub fn env_saw_path(&self) -> bool {
+        self.env.have_changed_path()
+    }
 
     pub fn setup_io(&self, default: Stdio, needs_stdin: bool)
                 -> io::Result<(StdioPipes, ChildPipes)> {

src/libstd/sys/unix/process/process_unix.rs

@@ -256,6 +256,7 @@ impl Command {
         if self.get_cwd().is_some() ||
             self.get_gid().is_some() ||
             self.get_uid().is_some() ||
+            self.env_saw_path() ||
             self.get_closures().len() != 0 {
             return Ok(None)
         }

src/libstd/sys_common/process.rs

@@ -47,6 +47,7 @@ impl EnvKey for DefaultEnvKey {}
 #[derive(Clone, Debug)]
 pub struct CommandEnv<K> {
     clear: bool,
+    saw_path: bool,
     vars: BTreeMap<K, Option<OsString>>
 }
 
@@ -54,6 +55,7 @@ impl<K: EnvKey> Default for CommandEnv<K> {
     fn default() -> Self {
         CommandEnv {
             clear: false,
+            saw_path: false,
             vars: Default::default()
         }
     }
@@ -108,9 +110,11 @@ impl<K: EnvKey> CommandEnv<K> {
 
     // The following functions build up changes
     pub fn set(&mut self, key: &OsStr, value: &OsStr) {
+        self.maybe_saw_path(&key);
         self.vars.insert(key.to_owned().into(), Some(value.to_owned()));
     }
     pub fn remove(&mut self, key: &OsStr) {
+        self.maybe_saw_path(&key);
         if self.clear {
             self.vars.remove(key);
         } else {
@@ -121,4 +125,12 @@ impl<K: EnvKey> CommandEnv<K> {
         self.clear = true;
         self.vars.clear();
     }
+    pub fn have_changed_path(&self) -> bool {
+        self.saw_path || self.clear
+    }
+    fn maybe_saw_path(&mut self, key: &OsStr) {
+        if !self.saw_path && key.to_os_string() == OsString::from("PATH") {
+            self.saw_path = true;
+        }
+    }
 }