[MLton] building mlton on Solaris 10

[Gergely Buday]

Regarding mllex/yacc, the test wanted to build them with the
compiled-from-source mlton, so I had to edit the makefiles to use the
sol8 binary one (I haven't tried to build mlton yet). Also, there was
a problem that the generated grm.sig and grm.sml files were not
writable. With this modifications the mllex/yacc/prof tests went
without problems.

- Gergely