detect libtool

[Peter Johansson]

Hello,

My apologies if this is more suited for libtool@

I'm writing a macro to provide some flags for users of our library. I 
now realized that I would like to assign the falgs different values 
depending on whether users are using libtool or not. What is the best 
way to detect whether libtool is used or not? Detected at autoconf or 
configure time doesn't really matter but I suppose autoconf is always 
preferable if possible.

Thanks,
Peter