Nice! Compiles and runs on OSX (though I still need to manually comment out #include <stdio.h> from unix/mpconfigport.h because of #1252)
Tab autocomplete working as expected.
Edit to add: tests/run-tests works also:

432 tests performed (15782 individual testcases)
432 tests passed
1 tests skipped: extra_coverage

This breaks builds for both mingw/msvc windows ports so maybe needs some #ifdefs to unbreak those until they are fixed (by providing unix_mphal.c or making it build). I thought at least mingw would have termios.h but appears it doesn't. Maybe @msiemens has an idea about the best way to fix this? For msvc I can probably come up with a winapi based solution.

We certainly want any readline support to be configurable, as @stinos suggests.

Otherwise looks good for initial version, I won't even ask why there're fgetc instead of fgets ;-).

Superseded by #1286.