Install Unison v2.40 in macOS 11+

main.sh

# Get Unison v2.40.x
git clone https://github.com/bcpierce00/unison.git
cd unison
git checkout 2.40

# Install opam (OCaml verion manager
brew install opam

# Download OCaml v4.06.1
opam init
opam switch create 4.06.1

# Patch files
# 1. In `src/mkProjectInfo.ml`: change `revisionString` from `"$Rev$"` to `"$Rev: 600$"`
# 2. In `src/Makefile`: Change `INSTALLDIR = /usr/local/bin/`
# 3. In `src/Makefile.OCaml`:
#    - Add `CAMLFLAGS+=-unsafe-string` before `INCLFLAGS`
#    - Change MINOSXVERSION to 11.3
# 4. In `src/system/system_generic.ml`, change to `let symlink s1 s2 = Unix.symlink s1 s2`

# Buildddddd
make UISTYLE=text install