Hello,
After Windows ICC support was added to 'compile' script, similar could be done to 'libtool.m4'.
Related patch added in attachment. It was tested for GMP builds using Windows ICC with <shared|static> layouts, and all builds tasks finished successfully. Additionally ensured, that it doesn't break builds using MSVC.
CC: Peter Rosin, as Author of commit (http://git.savannah.gnu.org/cgit/libtool.git/commit/?id=1ac3ced59db72099c6049e05b3643042fd5a9ea4),
CC: Jonathan L Peyton, as Author of commit (http://git.savannah.gnu.org/cgit/automake.git/commit/?id=c40e27e1c2a60f58e72e65d73d808f782d55494a),
in case they'll be interested to make review aor join this discussion.
Best,
Alexander