Bug 25664: Update libc version requirement for Rust

When merging the patches for #25310 the libc version requirement in
`GettingStartedRust.md` and `configure.ac` did not get updated to the
now needed 0.2.39.

configure.ac

@@ -275,7 +275,7 @@ fi
 AM_CONDITIONAL(USEPYTHON, [test "x$PYTHON" != "x"])
 
 dnl List all external rust crates we depend on here. Include the version
-rust_crates="libc-0.2.22"
+rust_crates="libc-0.2.39"
 AC_SUBST(rust_crates)
 
 ifdef([AC_C_FLEXIBLE_ARRAY_MEMBER], [

doc/HACKING/GettingStartedRust.md

@@ -65,7 +65,7 @@ fetching dependencies from Cargo or specifying a local directory.
 
 You'll need the following Rust dependencies (as of this writing):
 
-    libc==0.2.22
+    libc==0.2.39
 
 We vendor our Rust dependencies in a separate repo using
 [cargo-vendor](https://github.com/alexcrichton/cargo-vendor).  To use