Notes on building Poly/ML as Isabelle component
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* component skeleton:
  $ isabelle build_polyml_component -s sha1 component
* include full source (without symlink), for example:
  $ wget https://github.com/polyml/polyml/archive/master.zip
* libgmp on x86_64-darwin:
  https://github.com/Homebrew/homebrew-core/blob/master/Formula/gmp.rb
  https://gmplib.org/download/gmp/gmp-6.1.2.tar.xz
  ./configure --enable-cxx --build=core2-apple-darwin"$(uname -r)"
  make check
  make install
  isabelle build_polyml -m64 -s sha1 src --with-gmp LDFLAGS='-L/usr/local/lib' CPPFLAGS='-O3 -I/usr/local/include'