Go builds with clang, without cgo module
Sorry, I meant to use ENV.compiler
isl 0.12.1 with more formula cleanups.

* Fix URLs to use new upstream repo and use tag to track stable
  releases.
  - Fix homepage to use freecode site.
  - Fix head URL.
* Now that isl needs to bootstrap before compile.
  - Add :autoconf, :automake, :libtool as build dependencies, and run
    ./autogen.sh before configure.
* Enable verbose build output to help future debugging.
gfortran formula improvement.

* Disable isl and cloog version check to avoid break when they upgrade.
* Enable link-time-optimization (LTO).