When building on my Gentoo system with 'MLton 20180207' I get this error message:

[...]
mlton  -mlb-path-var 'SRC /home/sielenk/UrWeb/urweb/src' -mlb-path-var 'BUILD /home/sielenk/UrWeb/urweb/src' -output bin/urweb src/compiler.mlb
unhandled exception: Fail: strange mlb path var: SRC
make[1]: *** [Makefile:896: bin/urweb] Error 1
[...]

I was unable to resolve this problem with changing the quoting in the command line but finally managed to get it working again by using the mlb-path-map file.

Feel free to ignore this pull request, since I did not try it on any other system.
But I would like this to be discoverable closer to your repo in case someone stumbles on the same problem.

Best regards,
Marvin