arrow

GH-40248: [R] fallback to the correct libtool when we find a GNU one (#40259)

### Rationale for this change

On the CRAN build machines the GNU libtool is on the path in front of the macOS libtool. Though these are named the same thing, they are actually very different and don't actually appear to be substitutes

I checked on a non-developer's machine to see if `/usr/bin/libtool` exists, and it did. So I believe we _should_ be ok with this even if xcode / command line tools haven't been installed.

One note: it's possible that we could get the GNU libtool in link mode to work with the right incantation (something like `libtool --mode=link --tag=CXX ${cmake_compiler} -o ...` but when I tried this I kept getting symbol not found errors. Ultimately, I think any mac that we are on will have the apple-provided libtool, so decided to go the route of finding it. 

### What changes are included in this PR?

When we detect we are on a GNU libtool, we look to `/usr/bin/libtool` instead.

### Are these changes tested?

Yes. See a broken config failing at https://github.com/apache/arrow/pull/40259#issuecomment-1967762074 and then the next one passes

### Are there any user-facing changes?

We will remain on CRAN

**This PR contains a "Critical Fix".**
* GitHub Issue: #40248

Lead-authored-by: Jonathan Keane <jkeane@gmail.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Jonathan Keane <jkeane@gmail.com>