Setting up opam with musl-libc

opam-musl-libc.txt

There are a few steps to be followed when setting up opam to use musl-libc.

1. Install musl-tools - this adds the wrapper musl-gcc to the system.
2. export CC=musl-gcc
3. Any supporting C libraries will need to be compiled with musl and installed in a special prefix, eg

apt-get source libgmp-dev # get the source for libgmp
CC=musl-gcc ./configure --prefix=/usr/local/musl # make sure the libs and headers are compiled with the right compiler and end up in a special path

4. Export the following variables to make sure opam finds them
export CC=musl-cc
export C_INCLUDE_PATH=/usr/local/musl/include:.... # add whatever extra paths may be needed here
export LIBRARY_PATH=/usr/local/musl/lib

5. Install opam, create its switch: 
opam switch create 4.08.1+musl+static+flambda -v

Note: its a good idea to also set 
export OPAMEXTERNALSOLVER=mccs
as the default resolver can fail

6. switch over to the opam env
eval `opam env`

7. Now install using opam install etc