fix some errors in libstd

Author: mikeyhew

src/libstd/sys/unix/os.rs

@@ -453,7 +453,7 @@ pub fn getenv(k: &OsStr) -> io::Result<Option<OsString>> {
     let k = CString::new(k.as_bytes())?;
     unsafe {
         ENV_LOCK.lock();
-        let s = libc::getenv(k.as_ptr()) as *const _;
+        let s = libc::getenv(k.as_ptr()) as *const libc::c_char;
         let ret = if s.is_null() {
             None
         } else {

src/libstd/sys_common/gnu/libbacktrace.rs

@@ -73,7 +73,7 @@ pub fn resolve_symname<F>(frame: Frame,
                 "failed to allocate libbacktrace state")
             )
         }
-        let mut data = ptr::null();
+        let mut data: *const libc::c_char = ptr::null();
         let data_addr = &mut data as *mut *const libc::c_char;
         let ret = unsafe {
             backtrace_syminfo(state,