configure.ac: drop unused VOID_INT_SIGNALS

Summary:
Another macro borrowed from hugs, gone aways in
commit 528a7d2cf1c90408d60028bb1fec85124d539476

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

Test Plan: build-tested

Reviewers: simonmar, austin, ezyang

Reviewed By: austin, ezyang

Subscribers: phaskell, simonmar, relrod, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D118